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    SubjectRe: [Patch] KVM: x86/mmu: Make optimized __handle_changed_spte() for clear dirty log
    On Mon, Jan 30, 2023 at 3:49 PM Sean Christopherson <seanjc@google.com> wrote:
    >
    > On Mon, Jan 30, 2023, David Matlack wrote:
    > > On Wed, Jan 25, 2023 at 5:49 PM Sean Christopherson <seanjc@google.com> wrote:
    > > [...]
    > > > ---
    > > > arch/x86/kvm/mmu/tdp_mmu.c | 92 ++++++++++----------------------------
    > > > 1 file changed, 24 insertions(+), 68 deletions(-)
    > > >
    > > > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
    > > > index bba33aea0fb0..2f78ca43a276 100644
    > > > --- a/arch/x86/kvm/mmu/tdp_mmu.c
    > > > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
    > > [...]
    > > > @@ -1289,8 +1244,7 @@ static bool age_gfn_range(struct kvm *kvm, struct tdp_iter *iter,
    > > > new_spte = mark_spte_for_access_track(new_spte);
    > > > }
    > > >
    > > > - tdp_mmu_set_spte_no_acc_track(kvm, iter, new_spte);
    > > > -
    > > > + kvm_tdp_mmu_write_spte(iter->sptep, iter->old_spte, new_spte, iter->level);
    > >
    > > This can race with fast_page_fault() setting the W-bit and the CPU
    > > setting the D-bit. i.e. This call to kvm_tdp_mmu_write_spte() could
    > > clear the W-bit or D-bit.
    >
    > Ugh, right. Hrm. Duh, I just didn't go far enough. A straight XCHG is silly.
    > Except for the access-tracking mess, KVM wants to clear a single bit. Writing
    > the entire thing and potentially clobbering bits is wasteful and unnecessarily
    > dangerous. And the access-tracking code already needs special handling.
    >
    > We can just simplify this all by adding a helper to clear a single bit (and
    > maybe even use clear_bit() and __clear_bit() if we save the bit number for the
    > W/A/D bits and not just the mask). And if it weren't for EPT (different A/D
    > locations), we could even use static asserts to restrict the usage to the W/A/D
    > bits :-/ Oh well.
    >
    > E.g. this

    This patch looks good. Vipin can you incorporate this in your next version?

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