pci-assign terminates the guest upon pread() / pwrite() error?

In hw/kvm/pci-assign.c a pread() error part of assigned_dev_pci_read() result in a hw_error(). Similarly a pwrite() error part of assigned_dev_pci_write() also result in a hw_error().

Would there be a way to avoid terminating the guest for those cases? How about we deassign the device upon error?

In hw/kvm/pci-assign.c a pread() error part of assigned_dev_pci_read() result in a hw_error(). Similarly a pwrite() error part of assigned_dev_pci_write() also result in a hw_error().

Would there be a way to avoid terminating the guest for those cases? How about we deassign the device upon error?

Quoted 6 lines · Etienne Martineau

In hw/kvm/pci-assign.c a pread() error part of assigned_dev_pci_read() result in a hw_error(). Similarly a pwrite() error part of assigned_dev_pci_write() also result in a hw_error().

Would there be a way to avoid terminating the guest for those cases? How about we deassign the device upon error?

By terminating the guest we contain the error vs allowing the guest to continue running with invalid data. De-assigning the device is asynchronous and relies on guest involvement, so damage is potentially already done. Is this a theoretical problem or do you actually have hardware that hits this? Thanks,

Alex

Quoted 8 lines · Alex Williamson

Quoted 6 lines · Etienne Martineau

In hw/kvm/pci-assign.c a pread() error part of assigned_dev_pci_read() result in a hw_error(). Similarly a pwrite() error part of assigned_dev_pci_write() also result in a hw_error().

Would there be a way to avoid terminating the guest for those cases? How about we deassign the device upon error?

Quoted 8 lines

By terminating the guest we contain the error vs allowing the guest to continue running with invalid data. De-assigning the device is asynchronous and relies on guest involvement, so damage is potentially already done. Is this a theoretical problem or do you actually have hardware that hits this? Thanks,

Alex

This problem is in the context of a Hot-pluggable device assigned to the guest. If the guest rd/wr the config space at the same time than the device is physically taken out then the guest will terminate with hw_error().

Because this limits the availability of the guest I think we should try to recover instead. I don't see what other damage can happen since guest's MMIO access to the stale device will go nowhere?

Quoted 18 lines · Etienne Martineau

Quoted 8 lines · Alex Williamson

Quoted 6 lines · Etienne Martineau

In hw/kvm/pci-assign.c a pread() error part of assigned_dev_pci_read() result in a hw_error(). Similarly a pwrite() error part of assigned_dev_pci_write() also result in a hw_error().

Would there be a way to avoid terminating the guest for those cases? How about we deassign the device upon error?

Quoted 8 lines

By terminating the guest we contain the error vs allowing the guest to continue running with invalid data. De-assigning the device is asynchronous and relies on guest involvement, so damage is potentially already done. Is this a theoretical problem or do you actually have hardware that hits this? Thanks,

Alex

Quoted 8 lines

This problem is in the context of a Hot-pluggable device assigned to the guest. If the guest rd/wr the config space at the same time than the device is physically taken out then the guest will terminate with hw_error().

Because this limits the availability of the guest I think we should try to recover instead. I don't see what other damage can happen since guest's MMIO access to the stale device will go nowhere?

So you're looking at implementing surprise device removal? There's not just config space, there's slow bar access and mmap'd spaces to worry about too. What does going nowhere mean? If it means reads return -1 and the guest is trying to read the data portion of a packet from the network or an hba, we've now passed bad data to the guest. Thanks,

Alex

Quoted 28 lines · Alex Williamson

Quoted 18 lines · Etienne Martineau

Quoted 8 lines · Alex Williamson

Quoted 6 lines · Etienne Martineau

In hw/kvm/pci-assign.c a pread() error part of assigned_dev_pci_read() result in a hw_error(). Similarly a pwrite() error part of assigned_dev_pci_write() also result in a hw_error().

Would there be a way to avoid terminating the guest for those cases? How about we deassign the device upon error?

Quoted 8 lines

By terminating the guest we contain the error vs allowing the guest to continue running with invalid data. De-assigning the device is asynchronous and relies on guest involvement, so damage is potentially already done. Is this a theoretical problem or do you actually have hardware that hits this? Thanks,

Alex

Quoted 8 lines

This problem is in the context of a Hot-pluggable device assigned to the guest. If the guest rd/wr the config space at the same time than the device is physically taken out then the guest will terminate with hw_error().

Because this limits the availability of the guest I think we should try to recover instead. I don't see what other damage can happen since guest's MMIO access to the stale device will go nowhere?

Quoted 10 lines

So you're looking at implementing surprise device removal? There's not just config space, there's slow bar access and mmap'd spaces to worry about too. What does going nowhere mean? If it means reads return -1 and the guest is trying to read the data portion of a packet from the network or an hba, we've now passed bad data to the guest. Thanks,

Alex

Thanks for your answer;

Yes we are doing 'surprise device removal' for assigned device. Note that the problem also exist with standard 'attention button' device removal.

The problem is all about fault isolation. Ideally, only the corresponding driver should be affected by this 'surprise device removal'. I think that taking down the guest is too coarse. Think about a 'surprise device removal' on the host. In that case the host is not taken down so why not do the same with the guest?

Yes some badness will be latched into the guest but really this not any different that having a mis-behaving device.

Quoted 42 lines · Etienne Martineau

Quoted 28 lines · Alex Williamson

Quoted 18 lines · Etienne Martineau

Quoted 8 lines · Alex Williamson

Quoted 6 lines · Etienne Martineau

In hw/kvm/pci-assign.c a pread() error part of assigned_dev_pci_read() result in a hw_error(). Similarly a pwrite() error part of assigned_dev_pci_write() also result in a hw_error().

Would there be a way to avoid terminating the guest for those cases? How about we deassign the device upon error?

Quoted 8 lines

By terminating the guest we contain the error vs allowing the guest to continue running with invalid data. De-assigning the device is asynchronous and relies on guest involvement, so damage is potentially already done. Is this a theoretical problem or do you actually have hardware that hits this? Thanks,

Alex

Quoted 8 lines

This problem is in the context of a Hot-pluggable device assigned to the guest. If the guest rd/wr the config space at the same time than the device is physically taken out then the guest will terminate with hw_error().

Because this limits the availability of the guest I think we should try to recover instead. I don't see what other damage can happen since guest's MMIO access to the stale device will go nowhere?

Quoted 10 lines

So you're looking at implementing surprise device removal? There's not just config space, there's slow bar access and mmap'd spaces to worry about too. What does going nowhere mean? If it means reads return -1 and the guest is trying to read the data portion of a packet from the network or an hba, we've now passed bad data to the guest. Thanks,

Alex

Thanks for your answer;

Quoted 8 lines

Yes we are doing 'surprise device removal' for assigned device. Note that the problem also exist with standard 'attention button' device removal.

The problem is all about fault isolation. Ideally, only the corresponding driver should be affected by this 'surprise device removal'. I think that taking down the guest is too coarse. Think about a 'surprise device removal' on the host. In that case the host is not taken down so why not do the same with the guest?

It depends on the host hardware. Some x86 hardware will try to isolate the fault with an NMI other architectures such as ia64 would pull a machine check on a driver access to unresponsive devices.

Quoted 2 lines

Yes some badness will be latched into the guest but really this not any different that having a mis-behaving device.

... which is a bad thing, but often undetectable. This is detectable. Thanks,

Alex

Quoted 56 lines · Alex Williamson

Quoted 42 lines · Etienne Martineau

Quoted 28 lines · Alex Williamson

Quoted 18 lines · Etienne Martineau

Quoted 8 lines · Alex Williamson

Quoted 6 lines · Etienne Martineau

In hw/kvm/pci-assign.c a pread() error part of assigned_dev_pci_read() result in a hw_error(). Similarly a pwrite() error part of assigned_dev_pci_write() also result in a hw_error().

Would there be a way to avoid terminating the guest for those cases? How about we deassign the device upon error?

Quoted 8 lines

By terminating the guest we contain the error vs allowing the guest to continue running with invalid data. De-assigning the device is asynchronous and relies on guest involvement, so damage is potentially already done. Is this a theoretical problem or do you actually have hardware that hits this? Thanks,

Alex

Quoted 8 lines

This problem is in the context of a Hot-pluggable device assigned to the guest. If the guest rd/wr the config space at the same time than the device is physically taken out then the guest will terminate with hw_error().

Because this limits the availability of the guest I think we should try to recover instead. I don't see what other damage can happen since guest's MMIO access to the stale device will go nowhere?

Quoted 10 lines

So you're looking at implementing surprise device removal? There's not just config space, there's slow bar access and mmap'd spaces to worry about too. What does going nowhere mean? If it means reads return -1 and the guest is trying to read the data portion of a packet from the network or an hba, we've now passed bad data to the guest. Thanks,

Alex

Thanks for your answer;

Quoted 8 lines

Yes we are doing 'surprise device removal' for assigned device. Note that the problem also exist with standard 'attention button' device removal.

The problem is all about fault isolation. Ideally, only the corresponding driver should be affected by this 'surprise device removal'. I think that taking down the guest is too coarse. Think about a 'surprise device removal' on the host. In that case the host is not taken down so why not do the same with the guest?

It depends on the host hardware. Some x86 hardware will try to isolate the fault with an NMI other architectures such as ia64 would pull a machine check on a driver access to unresponsive devices.

Quoted 8 lines

Quoted 2 lines

Yes some badness will be latched into the guest but really this not any different that having a mis-behaving device.

... which is a bad thing, but often undetectable. This is detectable. Thanks,

Alex

Our hardware is throwing a surprise link down PCIe AER and we are acting on it. I agree that for the generalized case NMI can be an issue.

Let me ask you that question. What would be the best way to support device removal (surprise or not) for guest assigned device then? How about signaling the guest from vfio_pci_remove()?

Quoted 66 lines · Etienne Martineau

Quoted 56 lines · Alex Williamson

Quoted 42 lines · Etienne Martineau

Quoted 28 lines · Alex Williamson

Quoted 18 lines · Etienne Martineau

Quoted 8 lines · Alex Williamson

Quoted 6 lines · Etienne Martineau

In hw/kvm/pci-assign.c a pread() error part of assigned_dev_pci_read() result in a hw_error(). Similarly a pwrite() error part of assigned_dev_pci_write() also result in a hw_error().

Would there be a way to avoid terminating the guest for those cases? How about we deassign the device upon error?

Quoted 8 lines

By terminating the guest we contain the error vs allowing the guest to continue running with invalid data. De-assigning the device is asynchronous and relies on guest involvement, so damage is potentially already done. Is this a theoretical problem or do you actually have hardware that hits this? Thanks,

Alex

Quoted 8 lines

This problem is in the context of a Hot-pluggable device assigned to the guest. If the guest rd/wr the config space at the same time than the device is physically taken out then the guest will terminate with hw_error().

Because this limits the availability of the guest I think we should try to recover instead. I don't see what other damage can happen since guest's MMIO access to the stale device will go nowhere?

Quoted 10 lines

So you're looking at implementing surprise device removal? There's not just config space, there's slow bar access and mmap'd spaces to worry about too. What does going nowhere mean? If it means reads return -1 and the guest is trying to read the data portion of a packet from the network or an hba, we've now passed bad data to the guest. Thanks,

Alex

Thanks for your answer;

Quoted 8 lines

Yes we are doing 'surprise device removal' for assigned device. Note that the problem also exist with standard 'attention button' device removal.

The problem is all about fault isolation. Ideally, only the corresponding driver should be affected by this 'surprise device removal'. I think that taking down the guest is too coarse. Think about a 'surprise device removal' on the host. In that case the host is not taken down so why not do the same with the guest?

It depends on the host hardware. Some x86 hardware will try to isolate the fault with an NMI other architectures such as ia64 would pull a machine check on a driver access to unresponsive devices.

Quoted 8 lines

Quoted 2 lines

Yes some badness will be latched into the guest but really this not any different that having a mis-behaving device.

... which is a bad thing, but often undetectable. This is detectable. Thanks,

Alex

Quoted 6 lines

Our hardware is throwing a surprise link down PCIe AER and we are acting on it. I agree that for the generalized case NMI can be an issue.

Let me ask you that question. What would be the best way to support device removal (surprise or not) for guest assigned device then? How about signaling the guest from vfio_pci_remove()?

Thanks for using vfio! :)

The 440fx chipset is really not designed to deal with these kinds of problems. Generally the best answer to "how should we expose foo to the guest" is to do it exactly like it is on the host. That means sending a surprise link down aer to the guest. That should be possible with q35.

We could potentially signal that in vfio_pci_remove, but we probably want to figure out how to relay the aer event to the guest and inject it into the emulated chipset.