Re: virtio-iommu hotplug issue

[Jean-Philippe Brucker]

Hello,

On Thu, Apr 13, 2023 at 01:49:43PM +0900, Akihiko Odaki wrote:
> Hi,
> 
> Recently I encountered a problem with the combination of Linux's
> virtio-iommu driver and QEMU when a SR-IOV virtual function gets disabled.
> I'd like to ask you what kind of solution is appropriate here and implement
> the solution if possible.
> 
> A PCIe device implementing the SR-IOV specification exports a virtual
> function, and the guest can enable or disable it at runtime by writing to a
> configuration register. This effectively looks like a PCI device is
> hotplugged for the guest.

Just so I understand this better: the guest gets a whole PCIe device PF
that implements SR-IOV, and so the guest can dynamically create VFs?  Out
of curiosity, is that a hardware device assigned to the guest with VFIO,
or a device emulated by QEMU?

> In such a case, the kernel assumes the endpoint is
> detached from the virtio-iommu domain, but QEMU actually does not detach it.
> 
> This inconsistent view of the removed device sometimes prevents the VM from
> correctly performing the following procedure, for example:
> 1. Enable a VF.
> 2. Disable the VF.
> 3. Open a vfio container.
> 4. Open the group which the PF belongs to.
> 5. Add the group to the vfio container.
> 6. Map some memory region.
> 7. Close the group.
> 8. Close the vfio container.
> 9. Repeat 3-8
> 
> When the VF gets disabled, the kernel assumes the endpoint is detached from
> the IOMMU domain, but QEMU actually doesn't detach it. Later, the domain
> will be reused in step 3-8.
> 
> In step 7, the PF will be detached, and the kernel thinks there is no
> endpoint attached and the mapping the domain holds is cleared, but the VF
> endpoint is still attached and the mapping is kept intact.
> 
> In step 9, the same domain will be reused again, and the kernel requests to
> create a new mapping, but it will conflict with the existing mapping and
> result in -EINVAL.
> 
> This problem can be fixed by either of:
> - requesting the detachment of the endpoint from the guest when the PCI
> device is unplugged (the VF is disabled)

Yes, I think this is an issue in the virtio-iommu driver, which should be
sending a DETACH request when the VF is disabled, likely from
viommu_release_device(). I'll work on a fix unless you would like to do it

> - detecting that the PCI device is gone and automatically detach it on
> QEMU-side.
> 
> It is not completely clear for me which solution is more appropriate as the
> virtio-iommu specification is written in a way independent of the endpoint
> mechanism and does not say what should be done when a PCI device is
> unplugged.

Yes, I'm not sure it's in scope for the specification, it's more about
software guidance

Thanks,
Jean