From: Sean Christopherson <seanjc@google.com>
To: "David Hildenbrand (Arm)" <david@kernel.org>
Cc: Ackerley Tng <ackerleytng@google.com>,
Paolo Bonzini <pbonzini@redhat.com>,
Thomas Gleixner <tglx@kernel.org>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
Ashish Kalra <ashish.kalra@amd.com>,
Michael Roth <michael.roth@amd.com>,
Brijesh Singh <brijesh.singh@amd.com>,
Marc Zyngier <maz@kernel.org>, Oliver Upton <oupton@kernel.org>,
Joey Gouly <joey.gouly@arm.com>,
Steffen Eiden <seiden@linux.ibm.com>,
Suzuki K Poulose <suzuki.poulose@arm.com>,
Zenghui Yu <yuzenghui@huawei.com>,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>, Fuad Tabba <tabba@google.com>,
Yan Zhao <yan.y.zhao@intel.com>,
Rick P Edgecombe <rick.p.edgecombe@intel.com>,
Vishal Annapurve <vannapurve@google.com>,
kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev
Subject: Re: [PATCH v2 0/4] Stop returning struct page from guest_memfd PFN lookup
Date: Tue, 18 Aug 2026 12:55:45 -0700 [thread overview]
Message-ID: <aoS4wd6Dy9DlluHl@google.com> (raw)
In-Reply-To: <807b4b9f-b1ca-4b5c-bc9e-22bf0637ba33@kernel.org>
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.
prev parent reply other threads:[~2026-08-18 19:55 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-08-18 9:15 [PATCH v2 0/4] Stop returning struct page from guest_memfd PFN lookup Ackerley Tng
2026-08-18 9:15 ` [PATCH v2 1/4] KVM: SEV: Treat unassigned RMP entry as benign race on PSMASH failure Ackerley Tng
2026-08-18 9:15 ` [PATCH v2 2/4] KVM: SEV: Drop page refcount early during RMP fault handling Ackerley Tng
2026-08-18 9:29 ` sashiko-bot
2026-08-18 9:15 ` [PATCH v2 3/4] KVM: SEV: Drop page refcount early in VMSA reload Ackerley Tng
2026-08-18 9:27 ` sashiko-bot
2026-08-18 9:15 ` [PATCH v2 4/4] KVM: guest_memfd: Stop returning struct page from PFN lookup Ackerley Tng
2026-08-18 9:31 ` sashiko-bot
2026-08-18 13:58 ` Suzuki K Poulose
2026-08-18 17:12 ` [PATCH v2 0/4] Stop returning struct page from guest_memfd " David Hildenbrand (Arm)
2026-08-18 19:55 ` Sean Christopherson [this message]
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