Re: [PATCH v2 2/6] KVM: x86/mmu: Properly account NX huge page workaround for nonpaging MMUs

Paolo Bonzini
2022-07-28T20:15:56Z

On 7/23/22 03:23, Sean Christopherson wrote:
> +
> + /*
> + * Note, enforcing the NX huge page mitigation for nonpaging MMUs
> + * (shadow paging, CR0.PG=0 in the guest) is completely unnecessary.
> + * The guest doesn't have any page tables to abuse and is guaranteed
> + * to switch to a different MMU when CR0.PG is toggled on (may not
> + * always be guaranteed when KVM is using TDP). See also make_spte().
> + */
> const bool nx_huge_page_workaround_enabled;
>
> /*
> diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
> index 7314d27d57a4..9f3e5af088a5 100644
> --- a/arch/x86/kvm/mmu/spte.c
> +++ b/arch/x86/kvm/mmu/spte.c
> @@ -147,6 +147,17 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
> if (!prefetch)
> spte |= spte_shadow_accessed_mask(spte);
>
> + /*
> + * For simplicity, enforce the NX huge page mitigation even if not
> + * strictly necessary. KVM could ignore if the mitigation if paging is
> + * disabled in the guest, but KVM would then have to ensure a new MMU
> + * is loaded (or all shadow pages zapped) when CR0.PG is toggled on,
> + * and that's a net negative for performance when TDP is enabled. KVM
> + * could ignore the mitigation if TDP is disabled and CR0.PG=0, as KVM
> + * will always switch to a new MMU if paging is enabled in the guest,
> + * but that adds complexity just to optimize a mode that is anything
> + * but performance critical.

Why even separate the two comments? I think they both belong in
make_spte().

Paolo