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New: nVMX: Support TPR shadow

Bug 54141 [email protected] 1 participant 4 messages Feb 20, 2013 – Apr 8, 2015 KVM

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#0 lore raw

https://bugzilla.kernel.org/show_bug.cgi?id=54141

Summary: nVMX: Support TPR shadow
Product: Virtualization
Version: unspecified
Platform: All
OS/Version: Linux
Tree: Mainline
Status: NEW
Severity: enhancement
Priority: P1
Component: kvm
AssignedTo: [email protected]
ReportedBy: [email protected]
Regression: No

Add support for TPR shadow, which may be important for performance of Windows L2 guests (which currently don't work anyway - see bug 53641). Note, however, that TPR shadow is an optional feature, and KVM (as L1) won't use it if not available to it.

Some of the things we'll need to do (I think):

1. Advertise CPU_BASED_TPR_SHADOW in MSR_IA32_VMX_PROCBASED_CTLS

2. In nested_vmx_exit_handled_cr, in "mov to cr8", consider if we need an "else if CPU_BASED_TPR_SHADOW" which sets the shadow and only exits if below the tpr_threshold. This may be unnecessary, because the processor will already do this if we put vmcs12 desires in vmcs02, and don't merge it with vmcs01. Also, do we need to change anything in "mov from cr8" in the same function? I don't think it will ever get called.

3. In prepare_vmcs02, set the TPR-shadow definitions from vmcs12, ignoring L0's wishes (if I understand correctly, this is the right thing to do):

4. In prepare_vmcs02, if nested_cpu_has(vmcs12, CPU_BASED_TPR_SHADOW), we need to set VIRTUAL_APIC_PAGE_ADDR. The code will look something like

        struct page *page =
            nested_get_page(vcpu, vmcs12->virtual_apic_page_addr);
        if (!page)
            return 1;
        vmcs_write64(VIRTUAL_APIC_PAGE_ADDR, page_to_phys(page));

but we need to save this "page" in vmx->nested and nested_release_page() it on nested_vmx_vmexit() or free_nested().

4. In prepare_vmcs02, set TPR_THRESHOLD as requested by L1. We used to have this code:

    if (vm_need_tpr_shadow(vcpu->kvm) &&
        nested_cpu_has(vmcs12, CPU_BASED_TPR_SHADOW))
        vmcs_write32(TPR_THRESHOLD, vmcs12->tpr_threshold);

But I need to consider if "vm_need_tpr_shadow" is the right thing to check.

5. In prepare_vmcs02, in setting up CPU_BASED_VM_EXEC_CONTROL, probably *leave* the code which removes the CPU_BASED_TPR_SHADOW from L0's exec_control (we'll get this bit from vmcs12's exec_control).

6. We used to have the following code in prepare_vmcs02, after exec_control |= vmcs12->cpu_based_vm_exec_control, to remove the TPR_SHADOW feature even if L1 asked for it in certain cases. I don't see why this was needed:

    if (!vm_need_tpr_shadow(vcpu->kvm) ||
        vmcs12->virtual_apic_page_addr == 0) {
        exec_control &= ~CPU_BASED_TPR_SHADOW;
#ifdef CONFIG_X86_64
        exec_control |= CPU_BASED_CR8_STORE_EXITING |
            CPU_BASED_CR8_LOAD_EXITING;
#endif
    } else if (exec_control & CPU_BASED_TPR_SHADOW) {
#ifdef CONFIG_X86_64
        exec_control &= ~CPU_BASED_CR8_STORE_EXITING;
        exec_control &= ~CPU_BASED_CR8_LOAD_EXITING;
#endif
    }
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author
#1 lore raw

[Bug 54141] nVMX: Support TPR shadow

https://bugzilla.kernel.org/show_bug.cgi?id=54141

Nadav Har'El <[email protected]> changed:

What |Removed |Added
----------------------------------------------------------------------------

             Blocks|                            |53601
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author
#3 lore raw

[Bug 54141] nVMX: Support TPR shadow

In reply to [email protected]

https://bugzilla.kernel.org/show_bug.cgi?id=54141

Paolo Bonzini <[email protected]> changed:

What |Removed |Added
----------------------------------------------------------------------------

             Status|NEW                         |RESOLVED
                 CC|                            |[email protected]
     Kernel Version|                            |3.19
         Resolution|---                         |CODE_FIX

--- Comment #1 from Paolo Bonzini <[email protected]> ---
Fixed by commit a7c0b07d5708 (KVM: nVMX: nested TPR shadow/threshold emulation, 2014-08-21).

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