Re: [PATCH v2 05/27] drm/i915/gvt: Verify VFIO-pinned page is THP when shadowing 2M gtt entry

[Sean Christopherson <seanjc@google.com>]

On Sat, May 06, 2023, Yan Zhao wrote:
> On Sat, May 06, 2023 at 02:35:41PM +0800, Yan Zhao wrote:
> > > > Maybe the checking of PageTransHuge(cur_page) and bailing out is not necessary.
> > > > If a page is not transparent huge, but there are 512 contigous 4K
> > > > pages, I think it's still good to map them in IOMMU in 2M.
> > > > See vfio_pin_map_dma() who does similar things.
> > > 
> > > I agree that bailing isn't strictly necessary, and processing "blindly" should
> > > Just Work for HugeTLB and other hugepage types.  I was going to argue that it
> > > would be safer to add this and then drop it at the end, but I think that's a
> > > specious argument.  If not checking the page type is unsafe, then the existing
> > > code is buggy, and this changelog literally states that the check for contiguous
> > > pages guards against any such problems.
> > > 
> > > I do think there's a (very, very theoretical) issue though.  For "CONFIG_SPARSEMEM=y
> > > && CONFIG_SPARSEMEM_VMEMMAP=n", struct pages aren't virtually contiguous with respect
> > > to their pfns, i.e. it's possible (again, very theoretically) that two struct pages
> > > could be virtually contiguous but physically discontiguous.  I suspect I'm being
> > > ridiculously paranoid, but for the efficient cases where pages are guaranteed to
> > > be contiguous, the extra page_to_pfn() checks should be optimized away by the
> > > compiler, i.e. there's no meaningful downside to the paranoia.
> > To make sure I understand it correctly:
> > There are 3 conditions:
> > (1) Two struct pages aren't virtually contiguous, but there PFNs are contiguous.
> > (2) Two struct pages are virtually contiguous but their PFNs aren't contiguous.
> >     (Looks this will not happen?)
> > (3) Two struct pages are virtually contiguous, and their PFNs are contiguous, too.
> >     But they have different backends, e.g.
> >     PFN 1 and PFN 2 are contiguous, while PFN 1 belongs to RAM, and PFN 2
> >     belongs to DEVMEM.
> > 
> > I think you mean condition (3) is problematic, am I right?
> Oh, I got it now.
> You are saying about condition (2), with "CONFIG_SPARSEMEM=y &&
> CONFIG_SPARSEMEM_VMEMMAP=n".
> Two struct pages are contiguous if one is at one section's tail and another at
> another section's head, but the two sections aren't for contiguous PFNs.

Yep, exactly.