Re: [PATCH v2 1/4] iommu/vt-d: Check before setting PGSNP bit in pasid table entry

From: Lu Baolu
Date: Sun Apr 24 2022 - 02:23:47 EST

On 2022/4/24 13:55, Tian, Kevin wrote:
From: Lu Baolu <>
Sent: Sunday, April 24, 2022 12:38 PM

On 2022/4/24 11:37, Tian, Kevin wrote:
This should be rebased on top of Jason's enforce coherency series
instead of blindly setting it. No matter whether it's legacy mode
where we set SNP in PTE or scalable mode where we set PGSNP
in PASID entry for entire page table, the trigger point should be
same i.e. when someone calls enforce_cache_coherency().
With Jason's enforce coherency series merged, we even don't need to set
PGSNP bit of a pasid entry for second level translation. 2nd level
always supports SNP in PTEs, so set PGSNP in pasid table entry is

Yes, this sounds correct for 2nd-level.

but setting PGSNP of 1st level translation is also relevant to that
change when talking about enforcing coherency in the guest. In
this case PASID_FLAG_PAGE_SNOOP should be set also after
enforce_cache_coherency() is called.

Yes. Agreed.

Currently it's always set for unmanaged domain in

if (domain->domain.type == IOMMU_DOMAIN_UNMANAGED)

Suppose we need a separate interface to update PGSNP after pasid
entry is set up.

Currently enforcing coherency is only used in VFIO. In the VFIO use
case, it's safe to always set PGSNP when an UNMANAGED domain is attached
on the first level pasid translation. We could add support of updating
PGSNP after pasid entry setup once there's a real need.

The real need is here. The iommu driver should not assume the
policy of VFIO, which is already communicated via the new
enforce_cache_coherency() interface. The same policy should
apply no matter whether 1st or 2nd level is in-use.

Okay, I think I will move this patch out of this series and put it in a
separated one for VT-d improvements after Jason's enforcing snoop series
gets merged. Thanks for your suggestions.

Best regards,