linux-kernel - Re: [PATCH 2/4] KVM: x86/tdp_mmu: Merge the prefetch into the is_access

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Message-ID: <Z6Yg0pORbMyC-9xA@google.com>
Date: Fri, 7 Feb 2025 07:03:46 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: pbonzini@...hat.com, rick.p.edgecombe@...el.com, 
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 2/4] KVM: x86/tdp_mmu: Merge the prefetch into the
 is_access_allowed() check

On Fri, Feb 07, 2025, Yan Zhao wrote:
> Merge the prefetch check into the is_access_allowed() check to determine a
> spurious fault.
> 
> In the TDP MMU, a spurious prefetch fault should also pass the
> is_access_allowed() check.

How so? 

  1. vCPU takes a write-fault on a swapped out page and queues an async #PF
  2. A different task installs a writable SPTE
  3. A third task write-protects the SPTE for dirty logging
  4. Async #PF handler faults in the SPTE, encounters a read-only SPTE for its
     write fault.

KVM shouldn't mark the gfn as dirty in this case.