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

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Date:   Sun, 24 Apr 2022 05:55:29 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>
CC:     "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2 1/4] iommu/vt-d: Check before setting PGSNP bit in
 pasid table entry

> From: Lu Baolu <baolu.lu@...ux.intel.com>
> 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.