Re: virtio-iommu hotplug issue

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
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Re: virtio-iommu hotplug issue

[Eric Auger]

Hi,

On 4/13/23 13:01, Akihiko Odaki wrote:
> On 2023/04/13 19:40, Jean-Philippe Brucker wrote:
>> 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?
>
> Yes, that's right. The guest can dynamically create and delete VFs.
> The device is emulated by QEMU: igb, an Intel NIC recently added to
> QEMU and projected to be released as part of QEMU 8.0.
From below description In understand you then bind this emulated device
to VFIO on guest, correct?
>
>>
>>> In such a case, the kernel assumes the endpoint is
>>> detached from the virtio-iommu domain, but QEMU actually does not
>>> detach it.
The QEMU virtio-iommu device executes commands from the virtio-iommu
driver and my understanding is the VFIO infra is not in trouble here. As
suggested by Jean, a detach command probably is missed.
>>>
>>> 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
>
> It will be nice if you prepare a fix. I will test your patch with my
> workload if you share it with me.

I can help testing too

Thanks

Eric
>
> Regards,
> Akihiko Odaki
>
>>
>>> - 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
>

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Re: virtio-iommu hotplug issue

[Jean-Philippe Brucker]

On Thu, Apr 13, 2023 at 08:01:54PM +0900, Akihiko Odaki wrote:
> Yes, that's right. The guest can dynamically create and delete VFs. The
> device is emulated by QEMU: igb, an Intel NIC recently added to QEMU and
> projected to be released as part of QEMU 8.0.

Ah great, that's really useful, I'll add it to my tests

> > 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
> 
> It will be nice if you prepare a fix. I will test your patch with my
> workload if you share it with me.

I sent a fix:
https://lore.kernel.org/linux-iommu/20230414150744.562456-1-jean-philippe@linaro.org/

Thank you for reporting this, it must have been annoying to debug

Thanks,
Jean

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