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

[Lu Baolu]

On 2022/4/24 13:55, Tian, Kevin wrote:
> > From: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> > 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
> > > > unnecessary.
> > > 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
> > > domain_setup_first_level():
> > >
> > > 	if (domain->domain.type == IOMMU_DOMAIN_UNMANAGED)
> > > 		flags |= PASID_FLAG_PAGE_SNOOP;
> > >
> > > 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,
baolu