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

[Jim Mattson]

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.

>                         kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(gpa));
>
>                 return true;
>
> base-commit: 31b4fc3bc64aadd660c5bfa5178c86a7ba61e0f7
> --
> 2.41.0.rc0.172.g3f132b7071-goog
>