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References: <20220513195000.99371-1-seanjc@google.com> <20220513195000.99371-3-seanjc@google.com>
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From:   Jim Mattson <jmattson@google.com>
Date:   Fri, 13 May 2022 13:27:25 -0700
Message-ID: <CALMp9eQn+QaK630apzOz-L-btHxGQuXcbEiovO8FLOtMmQp_CQ@mail.gmail.com>
Subject: Re: [PATCH 2/2] KVM: x86/mmu: Comment FNAME(sync_page) to document
 TLB flushing logic
To:     Sean Christopherson <seanjc@google.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, David Matlack <dmatlack@google.com>,
        Ben Gardon <bgardon@google.com>
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On Fri, May 13, 2022 at 12:50 PM Sean Christopherson <seanjc@google.com> wrote:
>
> Add a comment to FNAME(sync_page) to explain why the TLB flushing logic
> conspiculously doesn't handle the scenario of guest protections being
> reduced.  Specifically, if synchronizing a SPTE drops execute protections,
> KVM will not emit a TLB flush, whereas dropping writable or clearing A/D
> bits does trigger a flush via mmu_spte_update().  Architecturally, until
> the GPTE is implicitly or explicitly flushed from the guest's perspective,
> KVM is not required to flush any old, stale translations.
>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
Reviewed-by: Jim Mattson <jmattson@google.com>