Date: Wed, 29 Jul 2026 06:33:43 -0700
In-Reply-To: <99aef43760db560d32e12e99df6bf209c67260e8.camel@intel.com>
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References: <20260728002236.869865-1-seanjc@google.com> <99aef43760db560d32e12e99df6bf209c67260e8.camel@intel.com>
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Subject: Re: [PATCH 0/2] KVM: x86/mmu: Use LOCK CMPXCHG to clear Accessed bits
From: Sean Christopherson <seanjc@google.com>
To: Kai Huang <kai.huang@intel.com>
Cc: "pbonzini@redhat.com" <pbonzini@redhat.com>, "kvm@vger.kernel.org" <kvm@vger.kernel.org>, 
	"jthoughton@google.com" <jthoughton@google.com>, 
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Wed, Jul 29, 2026, Kai Huang wrote:
> On Mon, 2026-07-27 at 17:22 -0700, Sean Christopherson wrote:
> > I initially was planning on tweaking the FROZEN_SPTE value to make it ok to
> > use LOCK AND on Accessed bits, but AFAICT there's no meaningful downside to
> > using CMPXCHG.
> 
> Maybe do both, e.g., still remove the A-bit from FROZEN_SPTE?  AFAICT there's no
> good reason to keep the A-bit in FROZEN_SPTE.

Hmm, I'm leaning "no".

Once aging uses CMPXCHG, the A-bit isn't any more or less likely to be corrupted
than any other bits.  And arguably, using the A-bit is "safer", because (a) it's
got the same bit position on EPT and everything else, and (b) spuriously setting
or clearing the bit is more likely to be benign.