From: David Matlack <dmatlack@google.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Ben Gardon <bgardon@google.com>,
Vipin Sharma <vipinsh@google.com>,
pbonzini@redhat.com, kvm@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [Patch] KVM: x86/mmu: Make optimized __handle_changed_spte() for clear dirty log
Date: Tue, 31 Jan 2023 09:41:04 -0800 [thread overview]
Message-ID: <CALzav=cEoF4E3hYStpDDdi_RWvCYXF9iXvznbL234uuEQ0Zv=g@mail.gmail.com> (raw)
In-Reply-To: <Y9hXmz/nDOr1hQal@google.com>
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?
next prev parent reply other threads:[~2023-01-31 17:41 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-25 21:38 [Patch] KVM: x86/mmu: Make optimized __handle_changed_spte() for clear dirty log Vipin Sharma
2023-01-25 22:00 ` David Matlack
2023-01-25 22:25 ` Ben Gardon
2023-01-26 0:40 ` Vipin Sharma
2023-01-26 1:49 ` Sean Christopherson
2023-01-28 17:20 ` Vipin Sharma
2023-01-30 18:09 ` Sean Christopherson
2023-01-30 20:17 ` Vipin Sharma
2023-01-30 22:49 ` Sean Christopherson
2023-01-30 22:12 ` David Matlack
2023-01-30 23:49 ` Sean Christopherson
2023-01-31 17:41 ` David Matlack [this message]
2023-01-31 18:01 ` Vipin Sharma
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