Re: [PATCH RFC v1 3/5] KVM: guest_memfd: Call arch invalidation hooks when converting to shared

[Michael Roth <michael.roth@amd.com>]

On Tue, Jul 15, 2025 at 06:20:09AM -0700, Vishal Annapurve wrote:
> On Thu, Jun 12, 2025 at 5:56 PM Michael Roth <michael.roth@amd.com> wrote:
> >
> > When guest_memfd is used for both shared/private memory, converting
> > pages to shared may require kvm_arch_gmem_invalidate() to be issued to
> > return the pages to an architecturally-defined "shared" state if the
> > pages were previously allocated and transitioned to a private state via
> > kvm_arch_gmem_prepare().
> >
> > Handle this by issuing the appropriate kvm_arch_gmem_invalidate() calls
> > when converting ranges in the filemap to a shared state.
> >
> > Signed-off-by: Michael Roth <michael.roth@amd.com>
> > ---
> >  virt/kvm/guest_memfd.c | 22 ++++++++++++++++++++++
> >  1 file changed, 22 insertions(+)
> >
> > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> > index b77cdccd340e..f27e1f3962bb 100644
> > --- a/virt/kvm/guest_memfd.c
> > +++ b/virt/kvm/guest_memfd.c
> > @@ -203,6 +203,28 @@ static int kvm_gmem_shareability_apply(struct inode *inode,
> >         struct maple_tree *mt;
> >
> >         mt = &kvm_gmem_private(inode)->shareability;
> > +
> > +       /*
> > +        * If a folio has been allocated then it was possibly in a private
> > +        * state prior to conversion. Ensure arch invalidations are issued
> > +        * to return the folio to a normal/shared state as defined by the
> > +        * architecture before tracking it as shared in gmem.
> > +        */
> > +       if (m == SHAREABILITY_ALL) {
> > +               pgoff_t idx;
> > +
> > +               for (idx = work->start; idx < work->start + work->nr_pages; idx++) {
> 
> It is redundant to enter this loop for VM variants that don't need
> this loop e.g. for pKVM/TDX. I think KVM can dictate a set of rules
> (based on VM type) that guest_memfd will follow for memory management
> when it's created, e.g. something like:
> 1) needs pfn invalidation
> 2) needs zeroing on shared faults
> 3) needs zeroing on allocation

Makes sense. Maybe internal/reserved GUEST_MEMFD_FLAG_*'s that can be passed
to kvm_gmem_create()?

-Mike

> 
> > +                       struct folio *folio = filemap_lock_folio(inode->i_mapping, idx);
> > +
> > +                       if (!IS_ERR(folio)) {
> > +                               kvm_arch_gmem_invalidate(folio_pfn(folio),
> > +                                                        folio_pfn(folio) + folio_nr_pages(folio));
> > +                               folio_unlock(folio);
> > +                               folio_put(folio);
> > +                       }
> > +               }
> > +       }
> > +
> >         return kvm_gmem_shareability_store(mt, work->start, work->nr_pages, m);
> >  }
> >
> > --
> > 2.25.1
> >