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From:   Vipin Sharma <vipinsh@google.com>
Date:   Tue, 31 Jan 2023 10:01:05 -0800
Message-ID: <CAHVum0da7MBi7G4=jwxpjd51s4OZ_JD-u40e9KJteRLKKyej5w@mail.gmail.com>
Subject: Re: [Patch] KVM: x86/mmu: Make optimized __handle_changed_spte() for
 clear dirty log
To:     David Matlack <dmatlack@google.com>
Cc:     Sean Christopherson <seanjc@google.com>,
        Ben Gardon <bgardon@google.com>, pbonzini@redhat.com,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org
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On Tue, Jan 31, 2023 at 9:41 AM David Matlack <dmatlack@google.com> wrote:
>
> 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?

On it.