Re: [PATCH 6.1.y 1/2] KVM: x86/mmu: Use EMULTYPE flag to track write #PFs to shadow pages

[Sean Christopherson <seanjc@google.com>]

On Mon, Dec 08, 2025, Sasha Levin wrote:
> From: Sean Christopherson <seanjc@google.com>
> 
> [ Upstream commit 258d985f6eb360c9c7aacd025d0dbc080a59423f ]
> 
> Use a new EMULTYPE flag, EMULTYPE_WRITE_PF_TO_SP, to track page faults
> on self-changing writes to shadowed page tables instead of propagating
> that information to the emulator via a semi-persistent vCPU flag.  Using
> a flag in "struct kvm_vcpu_arch" is confusing, especially as implemented,
> as it's not at all obvious that clearing the flag only when emulation
> actually occurs is correct.
> 
> E.g. if KVM sets the flag and then retries the fault without ever getting
> to the emulator, the flag will be left set for future calls into the
> emulator.  But because the flag is consumed if and only if both
> EMULTYPE_PF and EMULTYPE_ALLOW_RETRY_PF are set, and because
> EMULTYPE_ALLOW_RETRY_PF is deliberately not set for direct MMUs, emulated
> MMIO, or while L2 is active, KVM avoids false positives on a stale flag
> since FNAME(page_fault) is guaranteed to be run and refresh the flag
> before it's ultimately consumed by the tail end of reexecute_instruction().
> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> Message-Id: <20230202182817.407394-2-seanjc@google.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> Stable-dep-of: 4da3768e1820 ("KVM: SVM: Don't skip unrelated instruction if INT3/INTO is replaced")
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---

Acked-by: Sean Christopherson <seanjc@google.com>