linux-kernel - Re: [PATCH] KVM: x86/mmu: Remove KVM MMU write lock when accessing indirect_shadow

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Date:   Mon, 5 Jun 2023 10:53:50 -0700
From:   Mingwei Zhang <mizhang@...gle.com>
To:     Ben Gardon <bgardon@...gle.com>
Cc:     Jim Mattson <jmattson@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/mmu: Remove KVM MMU write lock when accessing indirect_shadow_pages

On Mon, Jun 5, 2023 at 10:17 AM Ben Gardon <bgardon@...gle.com> wrote:
>
> On Mon, Jun 5, 2023 at 9:55 AM Jim Mattson <jmattson@...gle.com> wrote:
> >
> > On Sun, Jun 4, 2023 at 5:43 PM Mingwei Zhang <mizhang@...gle.com> wrote:
> > >
> > > Remove KVM MMU write lock when accessing indirect_shadow_pages counter when
> > > page role is direct because this counter value is used as a coarse-grained
> > > heuristics to check if there is nested guest active. Racing with this
> > > heuristics without mmu lock will be harmless because the corresponding
> > > indirect shadow sptes for the GPA will either be zapped by this thread or
> > > some other thread who has previously zapped all indirect shadow pages and
> > > makes the value to 0.
> > >
> > > Because of that, remove the KVM MMU write lock pair to potentially reduce
> > > the lock contension and improve the performance of nested VM. In addition
> > > opportunistically change the comment of 'direct mmu' to make the
> > > description consistent with other places.
> > >
> > > Reported-by: Jim Mattson <jmattson@...gle.com>
> > > Signed-off-by: Mingwei Zhang <mizhang@...gle.com>
> > > ---
> > >  arch/x86/kvm/x86.c | 10 ++--------
> > >  1 file changed, 2 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > index 5ad55ef71433..97cfa5a00ff2 100644
> > > --- a/arch/x86/kvm/x86.c
> > > +++ b/arch/x86/kvm/x86.c
> > > @@ -8585,15 +8585,9 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
> > >
> > >         kvm_release_pfn_clean(pfn);
> > >
> > > -       /* The instructions are well-emulated on direct mmu. */
> > > +       /* The instructions are well-emulated on Direct MMUs. */
>
> Nit: Internally within Google, on older kernels, we have the "Direct
> MMU" which was the precursor to the TDP MMU we all know and love. This
> comment however does not refer to the Direct MMU. Direct here just
> refers to the direct role bit being set. Since it's just descriptive,
> direct should not be capitalized in this comment, so no reason to
> change this line.

You are right., it is incorrect to uppercase the 'direct', since that
generates confusions with our internal MMU implementation. So, I will
just uppercase the 'mmu' here in the next version.