Re: [PATCH] KVM: x86/mmu: Remove KVM MMU write lock when accessing indirect_shadow_pages

[Mingwei Zhang]

On Mon, Jun 5, 2023 at 11:12 AM Jim Mattson <jmattson@xxxxxxxxxx> wrote:
>
> On Mon, Jun 5, 2023 at 10:42 AM Mingwei Zhang <mizhang@xxxxxxxxxx> wrote:
> >
> > On Mon, Jun 5, 2023 at 9:55 AM Jim Mattson <jmattson@xxxxxxxxxx> wrote:
> > >
> > > On Sun, Jun 4, 2023 at 5:43 PM Mingwei Zhang <mizhang@xxxxxxxxxx> 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@xxxxxxxxxx>
> > > > Signed-off-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
> > > > ---
> > > >  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. */
> > > >         if (vcpu->arch.mmu->root_role.direct) {
> > > > -               unsigned int indirect_shadow_pages;
> > > > -
> > > > -               write_lock(&vcpu->kvm->mmu_lock);
> > > > -               indirect_shadow_pages = vcpu->kvm->arch.indirect_shadow_pages;
> > > > -               write_unlock(&vcpu->kvm->mmu_lock);
> > > > -
> > > > -               if (indirect_shadow_pages)
> > > > +               if (READ_ONCE(vcpu->kvm->arch.indirect_shadow_pages))
> > >
> > > I don't understand the need for READ_ONCE() here. That implies that
> > > there is something tricky going on, and I don't think that's the case.
> >
> > READ_ONCE() is just telling the compiler not to remove the read. Since
> > this is reading a global variable,  the compiler might just read a
> > previous copy if the value has already been read into a local
> > variable. But that is not the case here...
>
> Not a global variable, actually, but that's not relevant. What would
> be wrong with using a previously read copy?

Nothing will be wrong I think since this is already just a heuristic.

>
> We don't always wrap reads in READ_ONCE(). It's actually pretty rare.
> So, there should be an explicit and meaningful reason.
>
> > Note I see there is another READ_ONCE for
> > kvm->arch.indirect_shadow_pages, so I am reusing the same thing.
>
> That's not a good reason. "If all of your friends jumped off a cliff,
> would you?"

:)

>
> > I did check the reordering issue but it should be fine because when
> > 'we' see indirect_shadow_pages as 0, the shadow pages must have
> > already been zapped. Not only because of the locking, but also the
> > program order in __kvm_mmu_prepare_zap_page() shows that it will zap
> > shadow pages first before updating the stats.

yeah, I forgot to mention that removing READ_ONCE() is ok for me.