linux-kernel - Re: [PATCH v8 03/11] KVM: x86/mmu: Factor out spte atomic bit clearing routine

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Message-ID: <CADrL8HUVD=0EZszqp4zzKXVCpm8tjTmy30+LiJR2nMcC-8PO9w@mail.gmail.com>
Date: Mon, 27 Jan 2025 11:51:46 -0800
From: James Houghton <jthoughton@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, David Matlack <dmatlack@...gle.com>, 
	David Rientjes <rientjes@...gle.com>, Marc Zyngier <maz@...nel.org>, 
	Oliver Upton <oliver.upton@...ux.dev>, Wei Xu <weixugc@...gle.com>, Yu Zhao <yuzhao@...gle.com>, 
	Axel Rasmussen <axelrasmussen@...gle.com>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 03/11] KVM: x86/mmu: Factor out spte atomic bit
 clearing routine

On Fri, Jan 10, 2025 at 2:34 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Tue, Nov 05, 2024, James Houghton wrote:
> > This new function, tdp_mmu_clear_spte_bits_atomic(), will be used in a
> > follow-up patch to enable lockless Accessed and R/W/X bit clearing.
>
> This is a lie.  tdp_mmu_clear_spte_bits_atomic() can only be used to clear the
> Accessed bit, clearing RWX bits for access-tracked SPTEs *must* be done with a
> CMPXCHG so that the original RWX protections are preserved.

I'm not sure why I wrote it like that. I've dropped the "and R/W/X" piece:

    This new function, tdp_mmu_clear_spte_bits_atomic(), will be used in a
    follow-up patch to enable lockless Accessed bit clearing.