Re: [PATCH 1/1] iommu/vt-d: Unconditionally flush device TLB for pasid table updates

[Baolu Lu]

On 2024/8/20 15:09, Yi Liu wrote:
> On 2024/8/20 11:02, Lu Baolu wrote:
> > The caching mode of an IOMMU is irrelevant to the behavior of the device
> > TLB. Previously, commit <304b3bde24b5> ("iommu/vt-d: Remove caching mode
> > check before device TLB flush") removed this redundant check in the
> > domain unmap path.
> >
> > Checking the caching mode before flushing the device TLB after a pasid
> > table entry is updated is unnecessary and can lead to inconsistent
> > behavior.
> >
> > Extends this consistency by removing the caching mode check in the pasid
> > table update path.
>
> I'm wondering if a fix tag is needed here. Before this patch, the guest
> kernel does not issue device TLB invalidation. This may be a problem for
> the emulated devices that support ATS capability. The cache in device
> side would be stale. Although some vIOMMU like QEMU virtual VT-d would
> notify the emulated devices to flush their cache when handling the unmap
> event. [1]. But this is not required by VT-d spec. So it's possible that
> other vIOMMU may not do it. So this patch appears to fix an issue, so a fix
> tag may be necessary. How about your thought?

It depends on whether this solves any real or potential issues.

Thanks,
baolu