Re: [PATCH v2 0/4] Stop returning struct page from guest_memfd PFN lookup

[Sean Christopherson <seanjc@google.com>]

On Wed, Aug 19, 2026, David Hildenbrand (Arm) wrote:
> On 8/18/26 21:55, Sean Christopherson wrote:
> > On Tue, Aug 18, 2026, David Hildenbrand (Arm) wrote:
> >> On 8/18/26 11:15, Ackerley Tng wrote:
> >>> KVM currently expects kvm_gmem_get_pfn() to return a refcounted struct
> >>> page. Callers (such as x86 TDP MMU, arm64 Stage-2 fault handler, and SEV-SNP
> >>> VMSA / RMP handlers) hold this refcount across page fault handling.
> >>>
> >>> Holding a page refcount across fault handling is problematic for guest_memfd.
> >>> In-place memory conversions between confidential computing shared and private
> >>> states inspect folio refcounts to ensure exclusive ownership by guest_memfd.  A
> >>> concurrent guest page fault taking a reference on the folio causes conversions
> >>> to fail due to an elevated refcount.
> >>
> >> Right. Won't we still, at least temporarily, grab a reference while looking up
> >> the folio in the page cache, or will we be preventing that concurrent race with
> >> locking?
> > 
> > The latter.  What I want to aim for is that if the relevant guest_memfd range
> > has never been mmap()'d and there are no memory failures, then conversion is
> > guaranteed to not fail due to elevated refcounts.
> > 
> > Or to put it a different way, I want KVM's ABI to be that pausing vCPU is *NOT*
> > required to perform an in-place conversion.
> 
> Having the VM access a page that is currently under conversion (triggered by the
> VM) should not be the common case, no? Except, prefaulting, of course.

"not be the common case" is likely an understatement.  In practice, I don't it
will happen outside of guest bugs and KVM testcases.

It's the testcases that I want to "unblock" though.  If we commit to never having
to pause vCPUs, even if the guest is misbehaving, then that gives us deterministic
behavior we can validate, i.e. a way to detect similar regressions in the future.

I don't expect any regressions would be super problematic, but being able to treat
any failed conversion as a KVM bug (for the curated setup) mitigates the risk of
death by a thousand cuts, i.e. reduces the risk of gradually degrading conversion
performance because more and more transient references being taken by KVM.