linux-kernel - Re: [Patch] KVM: x86/mmu: Make optimized __handle_changed

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Date:   Tue, 31 Jan 2023 09:41:04 -0800
From:   David Matlack <dmatlack@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Ben Gardon <bgardon@...gle.com>, Vipin Sharma <vipinsh@...gle.com>,
        pbonzini@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [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@...gle.com> wrote:
>
> On Mon, Jan 30, 2023, David Matlack wrote:
> > On Wed, Jan 25, 2023 at 5:49 PM Sean Christopherson <seanjc@...gle.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?