proposal: KVM: Orphaned VMs: The Caretaker approach for Live Update

Hi all,

TL;DR: Below is a proposal for the next step in Live Update
functionality: maintaining vCPU execution during a host kernel reboot via a "Caretaker" for orphaned VMs.

As cloud infrastructure continues to push toward zero-downtime host maintenance, extending the capabilities of kexec-based Live Update to minimize guest disruption is becoming increasingly critical.

I would greatly appreciate your thoughts on the overall architecture, the proposed ABI, and any security or hardware-specific edge cases we might have missed.

Background
==========

Currently, Live Update allows us to preserve hardware resources across a kexec boundary, enabling VMs to be re-attached and resumed on the new kernel. This resource preservation is orchestrated from the kernel side by the LUO (https://docs.kernel.org/core-api/liveupdate.html). This proposal outlines how we can extend this foundation to keep the VMs running rather than just suspended during the transition, significantly reducing the effective blackout experienced by the virtual machines.

Orphaned VMs
============

Definition: An Orphaned VM is a virtual machine actively executing guest instructions on isolated physical hardware, completely decoupled from a Host Operating System or a userspace Virtual Machine Monitor (VMM).

Historically, a VM's lifecycle has been strictly tied to a userspace VMM process. The Orphaned VM strategy breaks this dependency by separating resource ownership from active execution.

Before the old kernel shuts down, it uses the Live Update Orchestrator to maintain ownership of the guest's underlying resources. The LUO preserves the vmfd and vcpufd structures, guest memory, secondary page tables, and other critical KVM metadata required to successfully restore the VM state in the new kernel.

During the transition, the active execution of the VM is not managed by the host kernel. Instead, execution is handed off to a specialized bare-metal component: The Caretaker.

While the VM is "orphaned," it operates entirely outside of standard userspace and kernel space. The physical CPU (pCPU) hosting the VM is completely isolated from the Host OS. The vCPUs continue to execute guest instructions uninterrupted, and any VM Exits are trapped and handled exclusively by the Caretaker. This isolated execution continues while the host kernel reboots on separate management cores (or while the VMM restarts, if the Live Update is limited to a userspace component).

The Caretaker
=============

The Caretaker is a specialized, identity-mapped bare-metal executable attached to each vCPU. While it is installed during the initial VM setup and remains permanently enabled as an interpose layer between the Guest and KVM, it is the key architectural feature required for a VM to be orphaned. By providing an execution environment independent of the host OS, the Caretaker enables the guest workload to safely survive the host lifecycle transition.

During normal VM operation, the Caretaker acts as a fast-path shim: it forwards standard VM Exits down to the backing KVM kernel module and handles certain exits autonomously. However, during the "Management Gap" (when no host OS is active and standard KVM handlers are offline), the Caretaker enters a standalone mode to ensure continuous guest execution without host intervention.

While this proposal focuses on its critical role in minimally disruptive Live Update, the Caretaker is fundamentally designed as an extensible primitive. Its architecture allows it to be leveraged for a variety of other advanced virtualization use cases, such as running custom lightweight hypervisors or completely offloading virtualization duties to an accelerator card.

Constraints
-----------

The Caretaker is a bare-metal executable. It does not have a backing kernel, it makes zero syscalls, and it lacks access to underlying hardware devices or complex I/O components.

Hardware Setup & ABI Interface
==============================

To interpose the Caretaker between the guest and the host OS, the initialization sequence must load the bare-metal payload, rewire the physical CPU's virtualization structures, and establish a syscall-free communication channel.

API Design & Caretaker Installation
-----------------------------------

The Caretaker is installed early in the VM's lifecycle (e.g., shortly after KVM_CREATE_VM). To manage this, we introduce a new KVM ioctl (KVM_SET_CARETAKER) that configures the shim on the vcpufd (alternatively on vmfd). Userspace provides the Caretaker payload compiled as an ELF binary.

Using an ELF binary allows the Caretaker to be separated into distinct memory sections:

* .text: The executable bare-metal instructions. This section is mapped as read-execute (RX).
* .data / .rodata: The static data and variables. This includes the pre-populated per-vCPU metadata (such as CPUID topology responses) generated by the VMM.
* .ccb: A dedicated section specifically reserved for the Caretaker Control Block.

During installation, the userspace VMM opens the ELF file and passes a file descriptor to the KVM_SET_CARETAKER ioctl. By utilizing a structured ELF payload, the Caretaker's logic remains highly flexible, and allows the architecture to be utilized for broader features, such as injecting a custom lightweight hypervisor, or unconditionally forwarding VM exits to an accelerator card.

Hardware Interposition
----------------------

During the execution of the KVM_SET_CARETAKER ioctl, instead of pointing the hardware's return path to standard KVM entry points (e.g., vmx_vmexit or svm_vcpu_run), KVM reprograms the host-state return area of the CPU's hardware virtualization control structures (e.g., Intel VMCS, AMD VMCB, or ARM equivalent) to point directly into the bare-metal Caretaker environment.

Specifically, the following critical host-state fields (using x86 terminology for illustration) are updated:

* HOST_RIP: Set to point to the e_entry (entry point) of the provided ELF .text segment. Whenever the guest triggers a VM Exit, the CPU hardware will unconditionally jump to this address.
* HOST_RSP: Programmed to point to a specialized, pre-allocated bare-metal stack dedicated strictly to this vCPU's Caretaker. This allows it to execute completely independently of the host kernel's normal thread stacks.
* HOST_SSP and HOST_INTR_SSP_TABLE: KVM pre-allocates a Shadow Stack for the Caretaker. These VMCS fields are programmed during initialization to ensure that RET and IRET instructions executed by the Caretaker do not trigger fatal #CP faults when the host kernel is detached.
* HOST_GS_BASE: Programmed to point to the CCB/Shared Metadata for this vCPU.
* HOST_CR3: Configured to point to the Caretaker's isolated, identity-mapped page tables, ensuring memory fetch safety when the host kernel's page tables are torn down.

Note on Optimization vs. Security: Constantly switching the page table (CR3) on every VM Exit can be expensive due to TLB flushing. To optimize performance, the Caretaker can share the host kernel's page tables while the kernel is still around, and dynamically replace HOST_CR3 with the dedicated, isolated page tables only when the vCPU is orphaned (during the detachment phase). On the other hand, maintaining a permanently isolated CR3 for the Caretaker adds a strong security boundary, achieving hardware-enforced separation similar to KVM Address Space Isolation (ASI).

KVM-Caretaker ABI
=================

The Caretaker requires a defined ABI to communicate with the host KVM subsystem. This ABI is implemented via the shared, identity-mapped .ccb section of the ELF payload, acting as the Caretaker Control Block (CCB).

The CCB acts as the source of truth for the Caretaker's execution loop and contains three primary elements:

* Attachment State Flag: An atomic variable indicating the current relationship with the host KVM subsystem (e.g., KVM_ATTACHED or KVM_DETACHED).
* KVM Routing Pointers: The physical function pointers that the Caretaker uses to safely jump into the host KVM's standard VM Exit handlers when operating in normal mode.
* Shared Configuration Metadata: A physical pointer to dedicated memory pages used by the kernel to share dynamic vCPU configuration data with the Caretaker. Because every guest is configured

differently, KVM populates these pages with the specific parameters negotiated during VM initialization (such as CPUID feature masks, APIC routing, and timer states). These pages also include a pre-allocated Telemetry Buffer for the Caretaker to log VM Exits and spin-wait durations. These dedicated pages are explicitly preserved across the host reboot via KHO, ensuring the Caretaker maintains continuous access to the exact context required to accurately emulate trivial exits during the gap.

Caretaker VM Exit Flow
======================

Upon vmexit, the Caretaker's execution flow follows a routing hierarchy:

The Fast Path
-------------

The Caretaker first evaluates the VM Exit reason. If the exit belongs to a category that the Caretaker is programmed to resolve natively, it handles it internally. For example, profiling of guests has identified the following exit categories for potential local resolution:

* Guest Idle Exits (e.g., HLT): When the guest OS goes idle, it triggers idle exits. The Caretaker intercepts these and halts the physical core until the next guest-bound interrupt fires, preserving host power.
* Timer and APIC Exits: Even an idle guest frequently writes to interrupt controllers and system registers to configure internal timers. The Caretaker handles these trivial writes directly, acknowledging the timer updates.
* CPUID and System Registers: Exits like CPUID or safe system register accesses are resolved by reading pre-computed responses from the shared metadata pages to accurately service the guest dynamically.

Host Routing
------------

If the VM Exit requires KVM/VMM intervention (e.g., a Page Fault or emulated device I/O), the Caretaker cannot resolve it locally. It must check the CCB attachment state flag to determine where to route the exit:

* If KVM_ATTACHED: The host kernel is actively managing the system. The Caretaker acts as a fast-path trampoline, setting up the standard host registers and transferring execution to the KVM Routing Pointers.
* If KVM_DETACHED: The host kernel is offline for Live Update. The Caretaker places the specific vCPU into a safe "spin-wait" polling loop, continuously checking the CCB flag for a change.

Preservation of the vCPU
========================

For this orchestration to work across a host OS replacement, the file descriptor associated with the vCPU must outlive the userspace process that created it, leveraging the LUO:

* Creation: The VMM initially creates the vcpufd via the standard KVM_CREATE_VCPU ioctl on the VM-wide kvmfd. Immediately after creation, the VMM issues the KVM_SET_CARETAKER ioctl on the newly created vcpufd. This installs the Caretaker as an interpose layer, allocating the CCB and rewiring the hardware virtualization control structures (e.g., HOST_RIP) for this specific vCPU. A dedicated userspace thread then takes ownership of this file descriptor to drive the KVM_RUN loop.

* LUO Registration & Isolation: In preparation for minimally disruptive Live Update, the VMM acquires a session from the userspace luo-agent and registers the vcpufd using LIVEUPDATE_SESSION_PRESERVE_FD. KVM's LUO .preserve() handler is invoked. Prior to this step, the VMM must pause emulated devices; the guest relies on VFIO pass-through for primary I/O to survive the gap without triggering VMM-bound exits. Crucially, the .preserve() phase is the moment when the pCPU is isolated from the host OS. The kernel finalizes the state, offlines the core (from the OS perspective), transitions the vCPU fully into the Caretaker, and preserves the required KVM data to KHO.

* The Gap: When the VMM process exits prior to the kexec transition, open file descriptors are normally destroyed. However, because the vcpufd is registered with LUO, the kernel holds a reference to the underlying struct file to ensure it survives the reboot. While the VMM has exited, the isolated pCPU appears completely offline to the host OS. During the boot of the Next Kernel, the core smp_init() routine parses the LUO FLB data and explicitly skips initializing these preserved pCPUs. This shields the dedicated cores from reset signals, allowing the guest workload to continue uninterrupted.

* Reclamation: The Next Kernel boots and LUO deserializes the session. When the new VMM process spawns, it retrieves the preserved session and issues LIVEUPDATE_SESSION_RETRIEVE_FD using its token. LUO invokes KVM's .retrieve() callback to map the preserved vcpufd back into the new VMM's file descriptor table. As part of this retrieval process, the host formally brings the isolated pCPU back online, and the new VMM userspace thread is attached back to the active VM thread running on the vCPU. Finally, KVM populates the new KVM Routing Pointers in the CCB and atomically flips the Host State Flag back to KVM_ATTACHED. This breaks the Caretaker's spin-wait loop (if it is in this state), allowing standard KVM operation to resume.

Gap Observability and Telemetry
===============================

During the host transition standard host-level observability tools (e.g., ftrace, perf, eBPF) are completely offline. To ensure production readiness, site reliability, and precise latency accounting, the Caretaker must act as a standalone micro-profiler while the host is detached.

The architecture includes a pre-allocated, identity-mapped Telemetry Buffer, passed to the Caretaker via the Shared Configuration Metadata during installation. During the gap, the Caretaker writes to this memory region to record execution data:

* Exit Counters: The Caretaker increments hardware-specific counters for every VM Exit reason it encounters. This provides a precise tally of natively handled events (e.g., HLT, CPUID, or APIC writes) that occurred while the host was offline.

* Spin-Wait Profiling: If a complex exit forces the Caretaker to block, it reads the hardware time stamp counter (e.g., RDTSC on x86, CNTVCT_EL0 on ARM64) immediately before entering the spin-wait loop. It reads the counter again the exact moment the CCB flag flips back to KVM_ATTACHED. The Caretaker records the specific exit reason that caused the block and the total accumulated stall time.

* Post-Gap Ingestion: Upon successful reattachment, the newly booted host KVM subsystem parses this Telemetry Buffer. The recorded data is exported via debugfs (e.g., /sys/kernel/debug/kvm/).

Challenges During Gap
=====================

While the Caretaker manages standard VM Exits, completely offlining a physical CPU introduces critical edge cases regarding asynchronous hardware interrupts. Below are the primary architectural challenges and their proposed mitigations.

VFIO Interrupt Routing
----------------------

* The Problem: The design relies on VFIO pass-through for primary I/O to survive the gap. However, when a physical device (e.g., an NVMe drive) completes a read, it sends a hardware interrupt (MSI/MSI-X) back to the CPU. By default, external hardware interrupts cause a VM Exit. Because the Caretaker natively lacks Linux IRQ routing tables, it has no immediate mechanism to inject that specific device interrupt into the guest's virtual APIC or GIC. The guest would stall indefinitely waiting for the I/O completion.

* Proposed Solution: The deployment may utilize hardware-accelerated interrupt routing, such as Posted Interrupts (Intel PI), Advanced Virtual Interrupt Controller (AMD AVIC), or GICv4 Direct Virtual Interrupt Injection (ARM64). This hardware feature allows the IOMMU/SMMU to route physical device interrupts directly into the guest's virtual interrupt controller without causing a VM Exit. Alternatively, KVM could share its IRQ routing tables with the Caretaker via the Shared Configuration Metadata during setup. This would allow the Caretaker to intercept the hardware interrupt and manually inject the virtual interrupt into the guest in software.

Guest-to-Guest IPIs
-------------------

* The Problem: If the guest OS attempts to wake up a sleeping thread, one orphaned vCPU will send an Inter-Processor Interrupt (IPI) to another orphaned vCPU. In standard virtualization without hardware assistance, writing to the APIC ICR (or sending an ARM SGI) causes a VM Exit so the host KVM can emulate the message delivery. During the gap, KVM is unavailable to route this message.

* Proposed Solution: The architecture may leverage hardware virtualized interrupts (Intel APICv, AMD AVIC, or ARM GICv4.1 virtual SGIs). This allows the hardware silicon to handle IPI delivery between the isolated pCPUs natively, eliminating the VM Exit. Alternatively, the Caretaker can be programmed to emulate the IPI delivery. By utilizing the shared memory metadata, the Caretaker can determine the target vCPU and directly update its pending interrupt state.

Stray Hardware Interrupts
-------------------------

* The Problem: What happens if a Non-Maskable Interrupt (NMI), a hardware timer tick, or a Machine Check Exception / System Error (MCE / ARM SError) arrives while the CPU is actively executing Caretaker code in KVM_DETACHED mode?

* Proposed Solution: To safely handle these asynchronous events, the Caretaker payload should establish and load its own minimal, self-contained Interrupt Descriptor Table (IDT on x86) or Exception Vector Table (via VBAR_EL2 on ARM64) during initialization. This ensures that if a stray hardware event or NMI arrives while executing Caretaker instructions in host mode, the CPU can catch the fault and park the core in a known state rather than triggering a catastrophic host reboot.

On x86, when transitioning into the gap, KVM explicitly programs HOST_IDTR and HOST_GDTR to these self-contained tables. If an NMI or stray hardware event arrives, the CPU catches the fault using the Caretaker's native handlers, parking the core or logging the event rather than attempting host recovery. KVM also programs HOST_INTR_SSP_TABLE within the Caretaker's isolated environment so that if the exception handlers execute an IRET, the hardware's CET shadow stack unroll succeeds without triggering a #CP exception.

Timekeeping Drift
-----------------

* The Problem: While the guest is orphaned, it continues executing and reading time via the physical CPU's hardware Time Stamp Counter (TSC or ARM generic timer). Because the host kernel is offline, software-based paravirtualized clocks (such as kvmclock) are no longer being updated by host background threads. Furthermore, when the Next Kernel boots, its calculation of host CLOCK_MONOTONIC or wall-time might drift from the old kernel. If the new KVM subsystem resets the VM's TSC offsets or updates the PV clock structures with standard initialization values upon adoption, the guest could experience a time jump.

* Proposed Solution: During the gap, the guest relies entirely on the physical CPU's Invariant TSC (which the hardware automatically offsets natively via the VMCS/VMCB) for continuous timekeeping. To ensure safe reattachment, KVM must serialize the exact state of the guest's PV clocks, TSC offsets, and the old kernel's base reference times into KHO memory during the LUO .preserve() phase. Upon adoption, the new KVM subsystem must synchronize its internal tracking with this preserved data, ensuring that any subsequent updates to the guest's PV clock memory guarantee a strictly monotonic and smooth time progression.

Nested Page Table Updates
-------------------------

* The Problem: As the guest executes, it may attempt to access memory that has not yet been mapped by the hypervisor, or it may interact with MMIO regions. Normally, this triggers an EPT Violation (Intel) or NPT Page Fault (AMD), prompting KVM to allocate host pages and update the secondary page tables. How are these updates handled when the host KVM subsystem is offline during the gap?

* Proposed Solution: During the "Management Gap," there are absolutely no updates made to the NPT/EPT. The existing secondary page tables are fully preserved in memory via LUO kvmfd preservation prior to detachment, allowing the guest to seamlessly access all previously mapped memory. If the guest triggers a new page fault (requiring an NPT/EPT update) during the gap, the Caretaker simply categorizes it as a Blocking Exit.

Compromised Caretaker
---------------------

* The Problem: The Caretaker runs in Host Mode. If left unprotected, this could allow a lightly privileged userspace process (e.g., QEMU or crosvm) to inject arbitrary executable code directly into the CPU's most privileged hardware state (VMX Root / Ring 0 / EL2).

* Proposed Solution: To mitigate this risk, the KVM_SET_CARETAKER ioctl may adopt the security model used by the kexec_file_load() syscall. Rather than trusting userspace to pass physical addresses, the kernel must take full ownership of payload validation:

- In-Kernel ELF Parsing: Instead of passing raw segment addresses (which is vulnerable to manipulation), the VMM passes a file descriptor for the Caretaker binary. The host KVM subsystem then performs the ELF parsing entirely in-kernel. This guarantees that the kernel controls exactly where the .text, .data, and .ccb sections are mapped, preventing userspace from tricking the kernel into overwriting sensitive host memory.

- Signature Verification & Secure Boot: If the host is running with Secure Boot or Kernel Lockdown enabled, KVM mandates that the Caretaker ELF binary be cryptographically signed. The kernel verifies the signature against the system's trusted keyring (e.g., .builtin_trusted_keys) before loading it. An unsigned or modified payload is outright rejected.

- IMA (Integrity Measurement Architecture) Integration: The loading of the Caretaker is hooked directly into the kernel's IMA subsystem. The binary is measured (hashed and extended into the hardware TPM) for remote attestation and appraised against local security policies before execution is permitted.

Caretaker Update
----------------

* The Problem: Given that the Caretaker is permanently installed during VM setup, how does it get updated on long-running VMs?

* Proposed Solution: To update the Caretaker, we can only perform the update when vCPUs are not isolated (i.e., not during the live update gap):

- vCPU Quiescence: The userspace VMM issues the KVM_SET_CARETAKER ioctl. KVM then sends kvm_vcpu_kick to force the target vCPU to exit the guest and return to the host kernel.
- KVM parses the new ELF. It allocates fresh memory pages for the new .text and .data segments.
- The kernel populates the new .ccb with the existing VM context.
- After the new environment is fully staged, KVM updates the physical CPU's virtualization control structures.
- When the vCPU thread resumes and re-enters the guest, the next VM Exit will trigger the hardware to jump into the new Caretaker payload. The old memory segments are then safely freed.

I would prefer we only attach the whole caretaker and all of its specialties right around the point when live update happens. Why keep it dangling and active forever? That way you can also late load the kernel module that contains it, so you can be sure it's an up to date version.

Quoted 4 lines · Alexander Graf

I would prefer we only attach the whole caretaker and all of its specialties right around the point when live update happens. Why keep it dangling and active forever? That way you can also late load the kernel module that contains it, so you can be sure it's an up to date version.

"Why keep it dangling and active forever?"

I've always wanted to tie this to address space isolation.

The only way to truly stay in front of the constant stream of new speculation vulnerabilities has been to just make sure there's nothing sensitive accessible in the address space at all. Hence all the work on secret hiding, XPFO, proclocal, etc. — and hence the occasional researcher finding their shiny new (5-year-old) vulnerability and being confused when it doesn't leak anything *interesting* in certain environments.

I'd like to see the inner KVM_RUN loop switch to a completely separate address space, in which there's a kind of caretaker which can handle the bare minimum of interrupts and timers and the most common exits, and which *relatively* rarely has to come back into the real Linux address space.

And once you have that caretaker running in its own address space... why not just let it keep going while Linux does its kexec?

Quoted 80 lines · Pasha Tatashin

Hi all,

TL;DR: Below is a proposal for the next step in Live Update
functionality: maintaining vCPU execution during a host kernel reboot via a "Caretaker" for orphaned VMs.

As cloud infrastructure continues to push toward zero-downtime host maintenance, extending the capabilities of kexec-based Live Update to minimize guest disruption is becoming increasingly critical.

I would greatly appreciate your thoughts on the overall architecture, the proposed ABI, and any security or hardware-specific edge cases we might have missed.

Background
==========

Currently, Live Update allows us to preserve hardware resources across a kexec boundary, enabling VMs to be re-attached and resumed on the new kernel. This resource preservation is orchestrated from the kernel side by the LUO (https://docs.kernel.org/core-api/liveupdate.html). This proposal outlines how we can extend this foundation to keep the VMs running rather than just suspended during the transition, significantly reducing the effective blackout experienced by the virtual machines.

Orphaned VMs
============

Definition: An Orphaned VM is a virtual machine actively executing guest instructions on isolated physical hardware, completely decoupled from a Host Operating System or a userspace Virtual Machine Monitor (VMM).

Historically, a VM's lifecycle has been strictly tied to a userspace VMM process. The Orphaned VM strategy breaks this dependency by separating resource ownership from active execution.

Before the old kernel shuts down, it uses the Live Update Orchestrator to maintain ownership of the guest's underlying resources. The LUO preserves the vmfd and vcpufd structures, guest memory, secondary page tables, and other critical KVM metadata required to successfully restore the VM state in the new kernel.

During the transition, the active execution of the VM is not managed by the host kernel. Instead, execution is handed off to a specialized bare-metal component: The Caretaker.

While the VM is "orphaned," it operates entirely outside of standard userspace and kernel space. The physical CPU (pCPU) hosting the VM is completely isolated from the Host OS. The vCPUs continue to execute guest instructions uninterrupted, and any VM Exits are trapped and handled exclusively by the Caretaker. This isolated execution continues while the host kernel reboots on separate management cores (or while the VMM restarts, if the Live Update is limited to a userspace component).

The Caretaker
=============

The Caretaker is a specialized, identity-mapped bare-metal executable attached to each vCPU. While it is installed during the initial VM setup and remains permanently enabled as an interpose layer between the Guest and KVM, it is the key architectural feature required for a VM to be orphaned. By providing an execution environment independent of the host OS, the Caretaker enables the guest workload to safely survive the host lifecycle transition.

During normal VM operation, the Caretaker acts as a fast-path shim: it forwards standard VM Exits down to the backing KVM kernel module and handles certain exits autonomously. However, during the "Management Gap" (when no host OS is active and standard KVM handlers are offline), the Caretaker enters a standalone mode to ensure continuous guest execution without host intervention.

While this proposal focuses on its critical role in minimally disruptive Live Update, the Caretaker is fundamentally designed as an extensible primitive. Its architecture allows it to be leveraged for a variety of other advanced virtualization use cases, such as running custom lightweight hypervisors or completely offloading virtualization duties to an accelerator card.

Quoted 6 lines

Constraints
-----------

The Caretaker is a bare-metal executable. It does not have a backing kernel, it makes zero syscalls, and it lacks access to underlying hardware devices or complex I/O components.

I think you still want to define an ABI with the outer execution environment to allow primitives such as timers. The same the other way around: You want to define multiple entry points, so that you can for example have one that says "You're up for running this vCPU now" or "Stop all your work, serialize your state".

Quoted 11 lines

Hardware Setup & ABI Interface
==============================

To interpose the Caretaker between the guest and the host OS, the initialization sequence must load the bare-metal payload, rewire the physical CPU's virtualization structures, and establish a syscall-free communication channel.

Quoted 8 lines

API Design & Caretaker Installation
-----------------------------------

The Caretaker is installed early in the VM's lifecycle (e.g., shortly after KVM_CREATE_VM). To manage this, we introduce a new KVM ioctl (KVM_SET_CARETAKER) that configures the shim on the vcpufd (alternatively on vmfd). Userspace provides the Caretaker payload compiled as an ELF binary.

Yikes. So you get a random unprivileged user space injects binary code into the kernel primitive? Not great.

I think it would be better to think of the caretaker as a separate subsystem. Maybe even a kernel module that you load (that way it goes through the same signing logic as everything else). It can then take a KVM fd and cooperate in-kernel with KVM to do the hand-over.

Or alternatively you follow a model where the caretaker is always compiled in as part of KVM. That makes the outgoing ABI easier, but complicates the incoming path a bit.

Quoted 13 lines

Using an ELF binary allows the Caretaker to be separated into distinct memory sections:

* .text: The executable bare-metal instructions. This section is mapped as read-execute (RX).
* .data / .rodata: The static data and variables. This includes the pre-populated per-vCPU metadata (such as CPUID topology responses) generated by the VMM.
* .ccb: A dedicated section specifically reserved for the Caretaker Control Block.

Quoted 6 lines

During installation, the userspace VMM opens the ELF file and passes a file descriptor to the KVM_SET_CARETAKER ioctl. By utilizing a structured ELF payload, the Caretaker's logic remains highly flexible, and allows the architecture to be utilized for broader features, such as injecting a custom lightweight hypervisor, or unconditionally forwarding VM exits to an accelerator card.

Again, no way you want to have a random kernel binary load API :).

Quoted 31 lines

Hardware Interposition
----------------------

During the execution of the KVM_SET_CARETAKER ioctl, instead of pointing the hardware's return path to standard KVM entry points (e.g., vmx_vmexit or svm_vcpu_run), KVM reprograms the host-state return area of the CPU's hardware virtualization control structures (e.g., Intel VMCS, AMD VMCB, or ARM equivalent) to point directly into the bare-metal Caretaker environment.

Specifically, the following critical host-state fields (using x86 terminology for illustration) are updated:

* HOST_RIP: Set to point to the e_entry (entry point) of the provided ELF .text segment. Whenever the guest triggers a VM Exit, the CPU hardware will unconditionally jump to this address.
* HOST_RSP: Programmed to point to a specialized, pre-allocated bare-metal stack dedicated strictly to this vCPU's Caretaker. This allows it to execute completely independently of the host kernel's normal thread stacks.
* HOST_SSP and HOST_INTR_SSP_TABLE: KVM pre-allocates a Shadow Stack for the Caretaker. These VMCS fields are programmed during initialization to ensure that RET and IRET instructions executed by the Caretaker do not trigger fatal #CP faults when the host kernel is detached.
* HOST_GS_BASE: Programmed to point to the CCB/Shared Metadata for this vCPU.
* HOST_CR3: Configured to point to the Caretaker's isolated, identity-mapped page tables, ensuring memory fetch safety when the host kernel's page tables are torn down.

Quoted 9 lines

Note on Optimization vs. Security: Constantly switching the page table (CR3) on every VM Exit can be expensive due to TLB flushing. To optimize performance, the Caretaker can share the host kernel's page tables while the kernel is still around, and dynamically replace HOST_CR3 with the dedicated, isolated page tables only when the vCPU is orphaned (during the detachment phase). On the other hand, maintaining a permanently isolated CR3 for the Caretaker adds a strong security boundary, achieving hardware-enforced separation similar to KVM Address Space Isolation (ASI).

This means you're pulling the rug underneath Linux executing that code. How will this work in practice? Won't Linux get upset and scream that it can no longer access the CPU? What happens on IPI?

Quoted 22 lines

KVM-Caretaker ABI
=================

The Caretaker requires a defined ABI to communicate with the host KVM subsystem. This ABI is implemented via the shared, identity-mapped .ccb section of the ELF payload, acting as the Caretaker Control Block (CCB).

The CCB acts as the source of truth for the Caretaker's execution loop and contains three primary elements:

* Attachment State Flag: An atomic variable indicating the current relationship with the host KVM subsystem (e.g., KVM_ATTACHED or KVM_DETACHED).
* KVM Routing Pointers: The physical function pointers that the Caretaker uses to safely jump into the host KVM's standard VM Exit handlers when operating in normal mode.
* Shared Configuration Metadata: A physical pointer to dedicated memory pages used by the kernel to share dynamic vCPU configuration data with the Caretaker. Because every guest is configured

Quoted 8 lines

differently, KVM populates these pages with the specific parameters negotiated during VM initialization (such as CPUID feature masks, APIC routing, and timer states). These pages also include a pre-allocated Telemetry Buffer for the Caretaker to log VM Exits and spin-wait durations. These dedicated pages are explicitly preserved across the host reboot via KHO, ensuring the Caretaker maintains continuous access to the exact context required to accurately emulate trivial exits during the gap.

Why not create a clear handover point? You are either running with KVM and its user space vmm or you are running this "caretaker vmm" which then runs its own binary hypervisor thing. That way the handover can also be done fully through UAPI. You could even implement the same KVM API on a caretaker fd for example and just serialize/deserialize the vCPU state.

Or if we embed it into KVM proper, we can probably define a KHO representation of that state and reuse that. Then ingesting it on the incoming environment is the same problem you already have to solve with KHO compatibility.

Quoted 82 lines

Caretaker VM Exit Flow
======================

Upon vmexit, the Caretaker's execution flow follows a routing hierarchy:

The Fast Path
-------------

The Caretaker first evaluates the VM Exit reason. If the exit belongs to a category that the Caretaker is programmed to resolve natively, it handles it internally. For example, profiling of guests has identified the following exit categories for potential local resolution:

* Guest Idle Exits (e.g., HLT): When the guest OS goes idle, it triggers idle exits. The Caretaker intercepts these and halts the physical core until the next guest-bound interrupt fires, preserving host power.
* Timer and APIC Exits: Even an idle guest frequently writes to interrupt controllers and system registers to configure internal timers. The Caretaker handles these trivial writes directly, acknowledging the timer updates.
* CPUID and System Registers: Exits like CPUID or safe system register accesses are resolved by reading pre-computed responses from the shared metadata pages to accurately service the guest dynamically.

Host Routing
------------

If the VM Exit requires KVM/VMM intervention (e.g., a Page Fault or emulated device I/O), the Caretaker cannot resolve it locally. It must check the CCB attachment state flag to determine where to route the exit:

* If KVM_ATTACHED: The host kernel is actively managing the system. The Caretaker acts as a fast-path trampoline, setting up the standard host registers and transferring execution to the KVM Routing Pointers.
* If KVM_DETACHED: The host kernel is offline for Live Update. The Caretaker places the specific vCPU into a safe "spin-wait" polling loop, continuously checking the CCB flag for a change.

Preservation of the vCPU
========================

For this orchestration to work across a host OS replacement, the file descriptor associated with the vCPU must outlive the userspace process that created it, leveraging the LUO:

* Creation: The VMM initially creates the vcpufd via the standard KVM_CREATE_VCPU ioctl on the VM-wide kvmfd. Immediately after creation, the VMM issues the KVM_SET_CARETAKER ioctl on the newly created vcpufd. This installs the Caretaker as an interpose layer, allocating the CCB and rewiring the hardware virtualization control structures (e.g., HOST_RIP) for this specific vCPU. A dedicated userspace thread then takes ownership of this file descriptor to drive the KVM_RUN loop.

* LUO Registration & Isolation: In preparation for minimally disruptive Live Update, the VMM acquires a session from the userspace luo-agent and registers the vcpufd using LIVEUPDATE_SESSION_PRESERVE_FD. KVM's LUO .preserve() handler is invoked. Prior to this step, the VMM must pause emulated devices; the guest relies on VFIO pass-through for primary I/O to survive the gap without triggering VMM-bound exits. Crucially, the .preserve() phase is the moment when the pCPU is isolated from the host OS. The kernel finalizes the state, offlines the core (from the OS perspective), transitions the vCPU fully into the Caretaker, and preserves the required KVM data to KHO.

* The Gap: When the VMM process exits prior to the kexec transition, open file descriptors are normally destroyed. However, because the vcpufd is registered with LUO, the kernel holds a reference to the underlying struct file to ensure it survives the reboot. While the VMM has exited, the isolated pCPU appears completely offline to the host OS. During the boot of the Next Kernel, the core smp_init() routine parses the LUO FLB data and explicitly skips initializing these preserved pCPUs. This shields the dedicated cores from reset signals, allowing the guest workload to continue uninterrupted.

Quoted 12 lines

* Reclamation: The Next Kernel boots and LUO deserializes the session. When the new VMM process spawns, it retrieves the preserved session and issues LIVEUPDATE_SESSION_RETRIEVE_FD using its token. LUO invokes KVM's .retrieve() callback to map the preserved vcpufd back into the new VMM's file descriptor table. As part of this retrieval process, the host formally brings the isolated pCPU back online, and the new VMM userspace thread is attached back to the active VM thread running on the vCPU. Finally, KVM populates the new KVM Routing Pointers in the CCB and atomically flips the Host State Flag back to KVM_ATTACHED. This breaks the Caretaker's spin-wait loop (if it is in this state), allowing standard KVM operation to resume.

How does the caretaker continue execution across kexec?

Quoted 144 lines

Gap Observability and Telemetry
===============================

During the host transition standard host-level observability tools (e.g., ftrace, perf, eBPF) are completely offline. To ensure production readiness, site reliability, and precise latency accounting, the Caretaker must act as a standalone micro-profiler while the host is detached.

The architecture includes a pre-allocated, identity-mapped Telemetry Buffer, passed to the Caretaker via the Shared Configuration Metadata during installation. During the gap, the Caretaker writes to this memory region to record execution data:

* Exit Counters: The Caretaker increments hardware-specific counters for every VM Exit reason it encounters. This provides a precise tally of natively handled events (e.g., HLT, CPUID, or APIC writes) that occurred while the host was offline.

* Spin-Wait Profiling: If a complex exit forces the Caretaker to block, it reads the hardware time stamp counter (e.g., RDTSC on x86, CNTVCT_EL0 on ARM64) immediately before entering the spin-wait loop. It reads the counter again the exact moment the CCB flag flips back to KVM_ATTACHED. The Caretaker records the specific exit reason that caused the block and the total accumulated stall time.

* Post-Gap Ingestion: Upon successful reattachment, the newly booted host KVM subsystem parses this Telemetry Buffer. The recorded data is exported via debugfs (e.g., /sys/kernel/debug/kvm/).

Challenges During Gap
=====================

While the Caretaker manages standard VM Exits, completely offlining a physical CPU introduces critical edge cases regarding asynchronous hardware interrupts. Below are the primary architectural challenges and their proposed mitigations.

VFIO Interrupt Routing
----------------------

* The Problem: The design relies on VFIO pass-through for primary I/O to survive the gap. However, when a physical device (e.g., an NVMe drive) completes a read, it sends a hardware interrupt (MSI/MSI-X) back to the CPU. By default, external hardware interrupts cause a VM Exit. Because the Caretaker natively lacks Linux IRQ routing tables, it has no immediate mechanism to inject that specific device interrupt into the guest's virtual APIC or GIC. The guest would stall indefinitely waiting for the I/O completion.

* Proposed Solution: The deployment may utilize hardware-accelerated interrupt routing, such as Posted Interrupts (Intel PI), Advanced Virtual Interrupt Controller (AMD AVIC), or GICv4 Direct Virtual Interrupt Injection (ARM64). This hardware feature allows the IOMMU/SMMU to route physical device interrupts directly into the guest's virtual interrupt controller without causing a VM Exit. Alternatively, KVM could share its IRQ routing tables with the Caretaker via the Shared Configuration Metadata during setup. This would allow the Caretaker to intercept the hardware interrupt and manually inject the virtual interrupt into the guest in software.

Guest-to-Guest IPIs
-------------------

* The Problem: If the guest OS attempts to wake up a sleeping thread, one orphaned vCPU will send an Inter-Processor Interrupt (IPI) to another orphaned vCPU. In standard virtualization without hardware assistance, writing to the APIC ICR (or sending an ARM SGI) causes a VM Exit so the host KVM can emulate the message delivery. During the gap, KVM is unavailable to route this message.

* Proposed Solution: The architecture may leverage hardware virtualized interrupts (Intel APICv, AMD AVIC, or ARM GICv4.1 virtual SGIs). This allows the hardware silicon to handle IPI delivery between the isolated pCPUs natively, eliminating the VM Exit. Alternatively, the Caretaker can be programmed to emulate the IPI delivery. By utilizing the shared memory metadata, the Caretaker can determine the target vCPU and directly update its pending interrupt state.

Stray Hardware Interrupts
-------------------------

* The Problem: What happens if a Non-Maskable Interrupt (NMI), a hardware timer tick, or a Machine Check Exception / System Error (MCE / ARM SError) arrives while the CPU is actively executing Caretaker code in KVM_DETACHED mode?

* Proposed Solution: To safely handle these asynchronous events, the Caretaker payload should establish and load its own minimal, self-contained Interrupt Descriptor Table (IDT on x86) or Exception Vector Table (via VBAR_EL2 on ARM64) during initialization. This ensures that if a stray hardware event or NMI arrives while executing Caretaker instructions in host mode, the CPU can catch the fault and park the core in a known state rather than triggering a catastrophic host reboot.

On x86, when transitioning into the gap, KVM explicitly programs HOST_IDTR and HOST_GDTR to these self-contained tables. If an NMI or stray hardware event arrives, the CPU catches the fault using the Caretaker's native handlers, parking the core or logging the event rather than attempting host recovery. KVM also programs HOST_INTR_SSP_TABLE within the Caretaker's isolated environment so that if the exception handlers execute an IRET, the hardware's CET shadow stack unroll succeeds without triggering a #CP exception.

Timekeeping Drift
-----------------

* The Problem: While the guest is orphaned, it continues executing and reading time via the physical CPU's hardware Time Stamp Counter (TSC or ARM generic timer). Because the host kernel is offline, software-based paravirtualized clocks (such as kvmclock) are no longer being updated by host background threads. Furthermore, when the Next Kernel boots, its calculation of host CLOCK_MONOTONIC or wall-time might drift from the old kernel. If the new KVM subsystem resets the VM's TSC offsets or updates the PV clock structures with standard initialization values upon adoption, the guest could experience a time jump.

* Proposed Solution: During the gap, the guest relies entirely on the physical CPU's Invariant TSC (which the hardware automatically offsets natively via the VMCS/VMCB) for continuous timekeeping. To ensure safe reattachment, KVM must serialize the exact state of the guest's PV clocks, TSC offsets, and the old kernel's base reference times into KHO memory during the LUO .preserve() phase. Upon adoption, the new KVM subsystem must synchronize its internal tracking with this preserved data, ensuring that any subsequent updates to the guest's PV clock memory guarantee a strictly monotonic and smooth time progression.

Nested Page Table Updates
-------------------------

* The Problem: As the guest executes, it may attempt to access memory that has not yet been mapped by the hypervisor, or it may interact with MMIO regions. Normally, this triggers an EPT Violation (Intel) or NPT Page Fault (AMD), prompting KVM to allocate host pages and update the secondary page tables. How are these updates handled when the host KVM subsystem is offline during the gap?

Quoted 7 lines

* Proposed Solution: During the "Management Gap," there are absolutely no updates made to the NPT/EPT. The existing secondary page tables are fully preserved in memory via LUO kvmfd preservation prior to detachment, allowing the guest to seamlessly access all previously mapped memory. If the guest triggers a new page fault (requiring an NPT/EPT update) during the gap, the Caretaker simply categorizes it as a Blocking Exit.

You may want to prefault all nested page tables before you switch to detached mode.

Quoted 31 lines

Compromised Caretaker
---------------------

* The Problem: The Caretaker runs in Host Mode. If left unprotected, this could allow a lightly privileged userspace process (e.g., QEMU or crosvm) to inject arbitrary executable code directly into the CPU's most privileged hardware state (VMX Root / Ring 0 / EL2).

* Proposed Solution: To mitigate this risk, the KVM_SET_CARETAKER ioctl may adopt the security model used by the kexec_file_load() syscall. Rather than trusting userspace to pass physical addresses, the kernel must take full ownership of payload validation:

- In-Kernel ELF Parsing: Instead of passing raw segment addresses (which is vulnerable to manipulation), the VMM passes a file descriptor for the Caretaker binary. The host KVM subsystem then performs the ELF parsing entirely in-kernel. This guarantees that the kernel controls exactly where the .text, .data, and .ccb sections are mapped, preventing userspace from tricking the kernel into overwriting sensitive host memory.

- Signature Verification & Secure Boot: If the host is running with Secure Boot or Kernel Lockdown enabled, KVM mandates that the Caretaker ELF binary be cryptographically signed. The kernel verifies the signature against the system's trusted keyring (e.g., .builtin_trusted_keys) before loading it. An unsigned or modified payload is outright rejected.

Quoted 5 lines

- IMA (Integrity Measurement Architecture) Integration: The loading of the Caretaker is hooked directly into the kernel's IMA subsystem. The binary is measured (hashed and extended into the hardware TPM) for remote attestation and appraised against local security policies before execution is permitted.

We have all of this already. It's called "kernel module loader", no? :)

Quoted 12 lines

Caretaker Update
----------------

* The Problem: Given that the Caretaker is permanently installed during VM setup, how does it get updated on long-running VMs?

* Proposed Solution: To update the Caretaker, we can only perform the update when vCPUs are not isolated (i.e., not during the live update gap):

Quoted 11 lines

- vCPU Quiescence: The userspace VMM issues the KVM_SET_CARETAKER ioctl. KVM then sends kvm_vcpu_kick to force the target vCPU to exit the guest and return to the host kernel.
- KVM parses the new ELF. It allocates fresh memory pages for the new .text and .data segments.
- The kernel populates the new .ccb with the existing VM context.
- After the new environment is fully staged, KVM updates the physical CPU's virtualization control structures.
- When the vCPU thread resumes and re-enters the guest, the next VM Exit will trigger the hardware to jump into the new Caretaker payload. The old memory segments are then safely freed.

I would prefer we only attach the whole caretaker and all of its specialties right around the point when live update happens. Why keep it dangling and active forever? That way you can also late load the kernel module that contains it, so you can be sure it's an up to date version.

While this proposal focuses on its critical role in minimally disruptive Live Update, the Caretaker is fundamentally designed as an extensible primitive. Its architecture allows it to be leveraged for a variety of other advanced virtualization use cases, such as running custom lightweight hypervisors or completely offloading virtualization duties to an accelerator card.

During the execution of the KVM_SET_CARETAKER ioctl, instead of pointing the hardware's return path to standard KVM entry points (e.g., vmx_vmexit or svm_vcpu_run), KVM reprograms the host-state return area of the CPU's hardware virtualization control structures (e.g., Intel VMCS, AMD VMCB, or ARM equivalent) to point directly into the bare-metal Caretaker environment.

Note on Optimization vs. Security: Constantly switching the page table (CR3) on every VM Exit can be expensive due to TLB flushing. To optimize performance, the Caretaker can share the host kernel's page tables while the kernel is still around, and dynamically replace HOST_CR3 with the dedicated, isolated page tables only when the vCPU is orphaned (during the detachment phase). On the other hand, maintaining a permanently isolated CR3 for the Caretaker adds a strong security boundary, achieving hardware-enforced separation similar to KVM Address Space Isolation (ASI).

The Caretaker requires a defined ABI to communicate with the host KVM subsystem. This ABI is implemented via the shared, identity-mapped .ccb section of the ELF payload, acting as the Caretaker Control Block (CCB).

The CCB acts as the source of truth for the Caretaker's execution loop and contains three primary elements:

* Attachment State Flag: An atomic variable indicating the current relationship with the host KVM subsystem (e.g., KVM_ATTACHED or KVM_DETACHED).

* KVM Routing Pointers: The physical function pointers that the Caretaker uses to safely jump into the host KVM's standard VM Exit handlers when operating in normal mode.
* Shared Configuration Metadata: A physical pointer to dedicated memory pages used by the kernel to share dynamic vCPU configuration data with the Caretaker. Because every guest is configured

differently, KVM populates these pages with the specific parameters negotiated during VM initialization (such as CPUID feature masks, APIC routing, and timer states). These pages also include a pre-allocated Telemetry Buffer for the Caretaker to log VM Exits and spin-wait durations. These dedicated pages are explicitly preserved across the host reboot via KHO, ensuring the Caretaker maintains continuous access to the exact context required to accurately emulate trivial exits during the gap.

The Caretaker first evaluates the VM Exit reason. If the exit belongs to a category that the Caretaker is programmed to resolve natively, it handles it internally. For example, profiling of guests has identified the following exit categories for potential local resolution:

* Guest Idle Exits (e.g., HLT): When the guest OS goes idle, it triggers idle exits. The Caretaker intercepts these and halts the physical core until the next guest-bound interrupt fires, preserving host power.

* Timer and APIC Exits: Even an idle guest frequently writes to interrupt controllers and system registers to configure internal timers. The Caretaker handles these trivial writes directly, acknowledging the timer updates.

When the new VMM process spawns, it retrieves the
preserved session and issues LIVEUPDATE_SESSION_RETRIEVE_FD using its token. LUO invokes KVM's .retrieve() callback to map the preserved vcpufd back into the new VMM's file descriptor table. As part of this retrieval process, the host formally brings the isolated pCPU back online, and the new VMM userspace thread is attached back to the active VM thread running on the vCPU. Finally, KVM populates the new KVM Routing Pointers in the CCB and atomically flips the Host State Flag back to KVM_ATTACHED. This breaks the Caretaker's spin-wait loop (if it is in this state), allowing standard KVM operation to resume.

Guest-to-Guest IPIs
-------------------

* The Problem: If the guest OS attempts to wake up a sleeping thread, one orphaned vCPU will send an Inter-Processor Interrupt (IPI) to another orphaned vCPU. In standard virtualization without hardware assistance, writing to the APIC ICR (or sending an ARM SGI) causes a VM Exit so the host KVM can emulate the message delivery. During the gap, KVM is unavailable to route this message.

* Proposed Solution: The architecture may leverage hardware virtualized interrupts (Intel APICv, AMD AVIC, or ARM GICv4.1 virtual SGIs). This allows the hardware silicon to handle IPI delivery between the isolated pCPUs natively, eliminating the VM Exit. Alternatively, the Caretaker can be programmed to emulate the IPI delivery. By utilizing the shared memory metadata, the Caretaker can determine the target vCPU and directly update its pending interrupt state.

* The Problem: What happens if a Non-Maskable Interrupt (NMI), a hardware timer tick, or a Machine Check Exception / System Error (MCE / ARM SError) arrives while the CPU is actively executing Caretaker code in KVM_DETACHED mode?

* Proposed Solution: To safely handle these asynchronous events, [...] on x86, when transitioning into the gap, KVM explicitly programs HOST_IDTR and HOST_GDTR to [the caretaker's] tables.

* The Problem: As the guest executes, it may attempt to access memory that has not yet been mapped by the hypervisor, or it may interact with MMIO regions. Normally, this triggers an EPT Violation (Intel) or NPT Page Fault (AMD), prompting KVM to allocate host pages and update the secondary page tables. How are these updates handled when the host KVM subsystem is offline during the gap?

* Proposed Solution: During the "Management Gap," there are absolutely no updates made to the NPT/EPT. The existing secondary page tables are fully preserved in memory via LUO kvmfd preservation prior to detachment, allowing the guest to seamlessly access all previously mapped memory. If the guest triggers a new page fault (requiring an NPT/EPT update) during the gap, the Caretaker simply categorizes it as a Blocking Exit.

Compromised Caretaker
---------------------

* The Problem: The Caretaker runs in Host Mode. If left unprotected, this could allow a lightly privileged userspace process (e.g., QEMU or crosvm) to inject arbitrary executable code directly into the CPU's most privileged hardware state (VMX Root / Ring 0 / EL2).

* Proposed Solution: To mitigate this risk, the KVM_SET_CARETAKER ioctl may adopt the security model used by the kexec_file_load() syscall. Rather than trusting userspace to pass physical addresses, the kernel must take full ownership of payload validation:

Caretaker Update
----------------

* The Problem: Given that the Caretaker is permanently installed during VM setup, how does it get updated on long-running VMs?

I even wonder if, for long term simplicity, the interface for host->caretaker should be just for the caretaker to swallow the host into non-root mode, again as in Arm nVHE.

Quoted 3 lines · Paolo Bonzini

I even wonder if, for long term simplicity, the interface for host->caretaker should be just for the caretaker to swallow the host into non-root mode, again as in Arm nVHE.

There's a lot of merit in that approach.

I talked about wanting to use this 'caretaker' for secret hiding. But why have *voluntary* secret hiding with the kernel hiding things from its own address space, when you have have *mandatory* secret hiding with something running in EL2, like pKVM.

Honestly, I don't see the *caretaker* being much of an ABI at all, except from one kernel to the next.

The *userspace* ABI considerations are all about how you make a vCPU that runs asynchronously (should it conceptually just be an async KVM_RUN call, which allows the vCPU to run in a kernel thread up to the point of kexec? Why is it fundamentally tied to kexec at all?).

I'd love to start without kexec in the picture at all. Just show me the KVM API for starting a *confidential* guest (pKVM, SEV-SNP, whatever), leaving it running, completely stopping the VMM and then starting a new VMM to pick up from where it left off.

Quoted 5 lines · David Woodhouse

Quoted 3 lines · Paolo Bonzini

I even wonder if, for long term simplicity, the interface for host->caretaker should be just for the caretaker to swallow the host into non-root mode, again as in Arm nVHE.

Quoted 6 lines

There's a lot of merit in that approach.

I talked about wanting to use this 'caretaker' for secret hiding.  But why have *voluntary* secret hiding with the kernel hiding things from its own address space, when you have have *mandatory* secret hiding with something running in EL2, like pKVM.

Well, other than because it's a lot of work? :)

Quoted 4 lines

The *userspace* ABI considerations are all about how you make a vCPU that runs asynchronously (should it conceptually just be an async KVM_RUN call, which allows the vCPU to run in a kernel thread up to the point of kexec? Why is it fundamentally tied to kexec at all?).

It's not tied to kexec.  kexec is just forcing a handoff + forcing an update.

The big difference is that:

1) if you don't tie it to kexec, a detached vCPU thread is a struct vhost_task and a blocking vmexit schedules out the thread; while during kexec you have s/kthread/pCPU/ and halting the CPU instead of scheduling it out.

2) if you don't tie it to kexec, address space isolation is the only real reason for the complication of treating the caretaker as a separate bare metal program.  OTOH maybe that's a feature - you could do:

- ioctl(KVM_RUN_ASYNC)

- then vmfd/vcpufd handoff to a new mm on top

- then address space isolation on top

- then kexec (de)serialization on top

Quoted 4 lines

I'd love to start without kexec in the picture at all. Just show me the KVM API for starting a *confidential* guest (pKVM, SEV-SNP, whatever), leaving it running, completely stopping the VMM and then starting a new VMM to pick up from where it left off.

Why confidential?

Quoted 6 lines · Paolo Bonzini

Quoted 5 lines · David Woodhouse

Quoted 3 lines · Paolo Bonzini

I even wonder if, for long term simplicity, the interface for host->caretaker should be just for the caretaker to swallow the host into non-root mode, again as in Arm nVHE.

Quoted 8 lines

Quoted 6 lines

There's a lot of merit in that approach.

I talked about wanting to use this 'caretaker' for secret hiding.  But why have *voluntary* secret hiding with the kernel hiding things from its own address space, when you have have *mandatory* secret hiding with something running in EL2, like pKVM.

Well, other than because it's a lot of work? :)

If we avoided those things then we've never have any fun!

And in a week where there seems to be a new user-to-root exploit posted every day, the 'deprivilege the VMM and assume the guest has owned it' security model is looking rather scary. So the additional defence in depth of knowing that even *root* can't get the kernel to access other guests' memory might be the only thing that lets you sleep at night :)

Yes, it's a lot of work. But I think we've reached the point where mandatory secret hiding is... well... mandatory.

Quoted 8 lines

Quoted 4 lines

The *userspace* ABI considerations are all about how you make a vCPU that runs asynchronously (should it conceptually just be an async KVM_RUN call, which allows the vCPU to run in a kernel thread up to the point of kexec? Why is it fundamentally tied to kexec at all?).

It's not tied to kexec.  kexec is just forcing a handoff + forcing an update.

Quoted 6 lines

The big difference is that:

1) if you don't tie it to kexec, a detached vCPU thread is a struct vhost_task and a blocking vmexit schedules out the thread; while during kexec you have s/kthread/pCPU/ and halting the CPU instead of scheduling it out.

For now maybe. But "how does the caretaker do scheduling" is on definitely the list of future problems, for any environment where a physical host with N pCPUs is hosting >= N vCPUs.

(In the case of a true mandatory-secret-hiding caretaker at EL2, the scheduling part *could* be done by the residual purgatory-caretaker- thing at EL1 that all the secondary CPUs go to instead of being turned off. It would just be calling into EL2 to run the actual vCPUs. Thus leaving the EL2 code just to do its *one* job, which has the added benefit that the automated reasoning people put the knives down and no longer have that look in their eyes that they got when they thought you wanted to put a scheduler in their formally-proven EL2 code...)

Quoted 6 lines

2) if you don't tie it to kexec, address space isolation is the only real reason for the complication of treating the caretaker as a separate bare metal program.  OTOH maybe that's a feature - you could do:

- ioctl(KVM_RUN_ASYNC)

Quoted 1 line

- then vmfd/vcpufd handoff to a new mm on top

This much gives you a seamless upgrade of the userspace VMM without having to play fd-handover tricks. The old VMM detaches, the new one attaches. If you're quick, and the guests aren't doing much "admin" work but only passing traffic through passthrough PCI devices, the guests might not experience any non-negligible steal time at all.

Quoted 2 lines

- then address space isolation on top

Even voluntary secret hiding lets you sleep at night when the next Retbleed happens.

Quoted 1 line

- then kexec (de)serialization on top

... and this one is the holy grail.

So yes, that's exactly the kind of thing I was thinking, rather than trying to boil the ocean. There are sensible milestones along the way which give practical benefits.

But my point was *also* about understanding the actual userspace interface for this, even if we were to just focus on the live update and do it all in one amphetamine-and-tokens-fueled epic. What does it even look like, from the VMM point of view? How does the new VMM under the new kernel 'reattach' to the existing vCPUs?

I think we need the userspace API concepts for 'detach' and 'attach', including the permissions model for reattach, and we might as well implement and test them without the kexec in the middle to start with.

Quoted 7 lines

Quoted 5 lines

I'd love to start without kexec in the picture at all. Just show me the KVM API for starting a *confidential* guest (pKVM, SEV-SNP, whatever), leaving it running, completely stopping the VMM and then starting a new VMM to pick up from where it left off.

Why confidential?

Mostly so that confidential VMs aren't an *afterthought*, and the design of the detach/attach userspace ABI gets them right from the start.

I have some very similar observations to Alex and some very similar observations to David. This has to imply that everyone will agree with me. :)

Seriously, the main contention point, from reading the thread, is the placement and lifecycle of the caretaker. More on this later...

Quoted 6 lines · Pasha Tatashin

While this proposal focuses on its critical role in minimally disruptive Live Update, the Caretaker is fundamentally designed as an extensible primitive. Its architecture allows it to be leveraged for a variety of other advanced virtualization use cases, such as running custom lightweight hypervisors or completely offloading virtualization duties to an accelerator card.

One step at a time please---and as an initial step, just place it inside the kernel, a la Arm nVHE.

Since your design would have anyway the ability to update the caretaker, you can embed that part into the reattachment process, so that the new kernel can use its own caretaker.

This reduces a lot the need to establish a stable-ish ABI. Only the handover (kexec/LUO) needs to be stable, so that the new kernel can populate its kvm and kvm_vcpu structs. And for that we mostly have a solution
already: a stream of serialized ioctls.

Quoted 6 lines

During the execution of the KVM_SET_CARETAKER ioctl, instead of pointing the hardware's return path to standard KVM entry points (e.g., vmx_vmexit or svm_vcpu_run), KVM reprograms the host-state return area of the CPU's hardware virtualization control structures (e.g., Intel VMCS, AMD VMCB, or ARM equivalent) to point directly into the bare-metal Caretaker environment.

This can be done unconditionally for all VMs based on a module parameter, again as in Arm nVHE.

Quoted 10 lines

Note on Optimization vs. Security: Constantly switching the page table (CR3) on every VM Exit can be expensive due to TLB flushing. To optimize performance, the Caretaker can share the host kernel's page tables while the kernel is still around, and dynamically replace HOST_CR3 with the dedicated, isolated page tables only when the vCPU is orphaned (during the detachment phase). On the other hand, maintaining a permanently isolated CR3 for the Caretaker adds a strong security boundary, achieving hardware-enforced separation similar to KVM Address Space Isolation (ASI).

Agreed on this.

Quoted 8 lines

The Caretaker requires a defined ABI to communicate with the host KVM subsystem. This ABI is implemented via the shared, identity-mapped .ccb section of the ELF payload, acting as the Caretaker Control Block (CCB).

The CCB acts as the source of truth for the Caretaker's execution loop and contains three primary elements:

Quoted 3 lines

* Attachment State Flag: An atomic variable indicating the current relationship with the host KVM subsystem (e.g., KVM_ATTACHED or KVM_DETACHED).

This must be done atomically at the time Linux offlines/onlines a pCPU. The interface from Linux to the caretaker must use some kind of IPI so that the new kernel can force a VMEXIT (if needed) in the caretaker, ask it to serialize the vm state, and pass it down to the new kernel's caretaker.

Quoted 15 lines

* KVM Routing Pointers: The physical function pointers that the Caretaker uses to safely jump into the host KVM's standard VM Exit handlers when operating in normal mode.
* Shared Configuration Metadata: A physical pointer to dedicated memory pages used by the kernel to share dynamic vCPU configuration data with the Caretaker. Because every guest is configured differently, KVM populates these pages with the specific parameters negotiated during VM initialization (such as CPUID feature masks, APIC routing, and timer states). These pages also include a pre-allocated Telemetry Buffer for the Caretaker to log VM Exits and spin-wait durations. These dedicated pages are explicitly preserved across the host reboot via KHO, ensuring the Caretaker maintains continuous access to the exact context required to accurately emulate trivial exits during the gap.

All this is mostly unnecessary if the caretaker is provided by the kernel. The recently introduced remote ring buffers can be used for tracing too.

Quoted 9 lines

The Caretaker first evaluates the VM Exit reason. If the exit belongs to a category that the Caretaker is programmed to resolve natively, it handles it internally. For example, profiling of guests has identified the following exit categories for potential local resolution:

* Guest Idle Exits (e.g., HLT): When the guest OS goes idle, it triggers idle exits. The Caretaker intercepts these and halts the physical core until the next guest-bound interrupt fires, preserving host power.

I don't think HLT can be handled entirely here. Either you skip the exit completely or you have to go out to the scheduler. The HLT exit could be skipped unconditionally for an orphaned VM, but while there is a running kernel the caretaker has to run entirely with interrupts off and that limits what you can do.

In fact there is already a blueprint of what can be handled easily in the caretaker, namely vmx_exit_handlers_fastpath()/svm_exit_handlers_fastpath(). Stick to what exists already.

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* Timer and APIC Exits: Even an idle guest frequently writes to interrupt controllers and system registers to configure internal timers. The Caretaker handles these trivial writes directly, acknowledging the timer updates.

This depends heavily on the implementation of the hypervisor, for example it can be done on Intel via the preemption timer but not on AMD where an actual hrtimer is needed.

[...]

Quoted 11 lines

When the new VMM process spawns, it retrieves the
preserved session and issues LIVEUPDATE_SESSION_RETRIEVE_FD using its token. LUO invokes KVM's .retrieve() callback to map the preserved vcpufd back into the new VMM's file descriptor table. As part of this retrieval process, the host formally brings the isolated pCPU back online, and the new VMM userspace thread is attached back to the active VM thread running on the vCPU. Finally, KVM populates the new KVM Routing Pointers in the CCB and atomically flips the Host State Flag back to KVM_ATTACHED. This breaks the Caretaker's spin-wait loop (if it is in this state), allowing standard KVM operation to resume.

This would also include some kind of serialization of the old VM into the new kernel's struct kvm_vcpu.

Also some kind of feature negotiation is needed (if that fails, the VMs are terminated unceremoniously) so I believe that the transition into and out of the gap must be synchronous. For example with INIT/SIPI for the entry, and an IPI for the exit?

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Guest-to-Guest IPIs
-------------------

* The Problem: If the guest OS attempts to wake up a sleeping thread, one orphaned vCPU will send an Inter-Processor Interrupt (IPI) to another orphaned vCPU. In standard virtualization without hardware assistance, writing to the APIC ICR (or sending an ARM SGI) causes a VM Exit so the host KVM can emulate the message delivery. During the gap, KVM is unavailable to route this message.

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* Proposed Solution: The architecture may leverage hardware virtualized interrupts (Intel APICv, AMD AVIC, or ARM GICv4.1 virtual SGIs). This allows the hardware silicon to handle IPI delivery between the isolated pCPUs natively, eliminating the VM Exit. Alternatively, the Caretaker can be programmed to emulate the IPI delivery. By utilizing the shared memory metadata, the Caretaker can determine the target vCPU and directly update its pending interrupt state.

Yeah, I think APIC emulation to some extent must be moved into the VMX/SVM fastpaths. The good news is that this can be done already as a PoC without needing the whole caretaker and LUO infrastructure.

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* The Problem: What happens if a Non-Maskable Interrupt (NMI), a hardware timer tick, or a Machine Check Exception / System Error (MCE / ARM SError) arrives while the CPU is actively executing Caretaker code in KVM_DETACHED mode?

* Proposed Solution: To safely handle these asynchronous events, [...] on x86, when transitioning into the gap, KVM explicitly programs HOST_IDTR and HOST_GDTR to [the caretaker's] tables.

Agreed and this also shows that the transition must be synchronous.

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* The Problem: As the guest executes, it may attempt to access memory that has not yet been mapped by the hypervisor, or it may interact with MMIO regions. Normally, this triggers an EPT Violation (Intel) or NPT Page Fault (AMD), prompting KVM to allocate host pages and update the secondary page tables. How are these updates handled when the host KVM subsystem is offline during the gap?

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* Proposed Solution: During the "Management Gap," there are absolutely no updates made to the NPT/EPT. The existing secondary page tables are fully preserved in memory via LUO kvmfd preservation prior to detachment, allowing the guest to seamlessly access all previously mapped memory. If the guest triggers a new page fault (requiring an NPT/EPT update) during the gap, the Caretaker simply categorizes it as a Blocking Exit.

Yes, by default everything is a blocking exit. In particular, unless one day we do x86/pKVM, page tables can be handled entirely by Linux rather than the caretaker with no change to the existing MMU notifier architecture.

As a consequence, the caretaker is absolutely not going to be a TCB---at least not in the beginning.

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Compromised Caretaker
---------------------

* The Problem: The Caretaker runs in Host Mode. If left unprotected, this could allow a lightly privileged userspace process (e.g., QEMU or crosvm) to inject arbitrary executable code directly into the CPU's most privileged hardware state (VMX Root / Ring 0 / EL2).

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* Proposed Solution: To mitigate this risk, the KVM_SET_CARETAKER ioctl may adopt the security model used by the kexec_file_load() syscall. Rather than trusting userspace to pass physical addresses, the kernel must take full ownership of payload validation:

-EOVERENGINEERED. Just shove it into the kernel.

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Caretaker Update
----------------

* The Problem: Given that the Caretaker is permanently installed during VM setup, how does it get updated on long-running VMs?

Via kexec. :) I understand you have bigger plans, but we need to crawl before walk^Wattempting a marathon.

I even wonder if, for long term simplicity, the interface for host->caretaker should be just for the caretaker to swallow the host into non-root mode, again as in Arm nVHE. That would make it much harder to implement some kind of live update, but my answer to that *really* is just to use kexec.

Paolo

The way I see it, vmfd and vcpufd need to support LUO preservation by implementing the liveupdate_file_ops callbacks (.preserve, .restore).

When userspace preserves the vcpufd, the kernel isolates the assigned pCPU (probably requiring the vCPU to be pinned to the CPU, and the CPU to be in an isolated cpuset),

Even for this simplest form, we still need a defined ABI between the host kernel and the Caretaker. The host must send an IPI to synchronously notify the Caretaker of attach and detach transitions.

This ABI must also handle Caretaker replacement during the adoption
phase: when the new kernel retrieves the vCPU, it requires a protocol to notify the running Caretaker that the pCPU is being onlined. The Caretaker must reach a known, state at that moment so the kernel can seamlessly replace the previous kernel's Caretaker version with the current one. Finally, the Caretaker still relies on the CCB to access KVM routing pointers for forwarding VM exits back to the host during normal operation.

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This must be done atomically at the time Linux offlines/onlines a pCPU. The interface from Linux to the caretaker must use some kind of IPI so that the new kernel can force a VMEXIT (if needed) in the caretaker, ask it to serialize the vm state, and pass it down to the new kernel's caretaker.

Yes, agree.

During the kexec gap (when the host kernel is completely offline), the Caretaker acts purely as a producer, writing trace events directly to this physical memory block and advancing the ring buffer pointers.

When the new kernel boots and re-adopts the orphaned vCPU, it retrieves this memory from KHO and attaches it back to the tracing subsystem.

I agree regarding HLT: during the gap, the simplest approach is to just skip the exit and return directly to the VM without attempting to handle it.

I completely agree that the transition into and out of the gap must be synchronous. As discussed above, using an IPI is the right approach. For entry, the host kernel signals the Caretaker via IPI to ensure it reaches a known state before the pCPU is offlined.

For exit, the new
kernel sends an IPI to the orphaned pCPU to force a VM exit, allowing the new kernel to take control, update the Caretaker environment, and complete the reattachment.

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Yeah, I think APIC emulation to some extent must be moved into the VMX/SVM fastpaths. The good news is that this can be done already as a PoC without needing the whole caretaker and LUO infrastructure.

Moving APIC emulation into the VMX/SVM fastpaths makes sense as a standalone effort.

[...snip...]

The Caretaker
=============

The Caretaker is a specialized, identity-mapped bare-metal executable attached to each vCPU. While it is installed during the initial VM setup and remains permanently enabled as an interpose layer between the Guest and KVM, it is the key architectural feature required for a VM to be orphaned. By providing an execution environment independent of the host OS, the Caretaker enables the guest workload to safely survive the host lifecycle transition.

During normal VM operation, the Caretaker acts as a fast-path shim: it forwards standard VM Exits down to the backing KVM kernel module and handles certain exits autonomously. However, during the "Management Gap" (when no host OS is active and standard KVM handlers are offline), the Caretaker enters a standalone mode to ensure continuous guest execution without host intervention.

While this proposal focuses on its critical role in minimally disruptive Live Update, the Caretaker is fundamentally designed as an extensible primitive.

Its architecture allows it to be leveraged for a variety of other advanced virtualization use cases, such as running custom lightweight hypervisors or completely offloading virtualization duties to an accelerator card.

[...snip...]