Re: [PATCH 2/2] iommu/vt-d: Remove caching mode check before devtlb flush

Jacob Pan
2024-04-11T16:13:22Z

Hi Kevin,

On Wed, 10 Apr 2024 23:23:57 +0000, "Tian, Kevin" <kevin.tian@intel.com>
wrote:

> > From: Jacob Pan <jacob.jun.pan@linux.intel.com>
> > Sent: Thursday, April 11, 2024 12:20 AM
> >
> > Hi Kevin,
> >
> > On Wed, 10 Apr 2024 00:32:06 +0000, "Tian, Kevin" <kevin.tian@intel.com>
> > wrote:
> >
> > > > From: Jacob Pan <jacob.jun.pan@linux.intel.com>
> > > > Sent: Wednesday, April 10, 2024 1:32 AM
> > > >
> > > > If the guest uses SL page tables in vIOMMU, we don;t expose ATS to
> > > > the guest. So ATS is not relevant here, does't matter map or unmap.
> > > >
> > >
> > > ATS is orthogonal to SL vs. FL. Where is this restriction coming
> > > from?
> > For practical purposes, what would be the usage to have SL in the guest
> > and ATS enabled. i.e. shadowing SL but directly expose ATS?
> >
>
> ATS is about the protocol between device and iommu to look up
> translations. Why does it care about internal paging layout in
> iommu?
>
Maybe the original intent was missed, I was suggesting the devTLB flush
should be based on ATS cap (as you said here) not map/unmap.

- /*
- * In caching mode, changes of pages from non-present to present require
- * flush. However, device IOTLB doesn't need to be flushed in this case.
- */
- if (!cap_caching_mode(iommu->cap) || !map)
- iommu_flush_dev_iotlb(domain, addr, mask);
+ iommu_flush_dev_iotlb(domain, addr, mask);

Thanks,

Jacob