[PATCH v4 15/20] KVM: x86/mmu: Cache the access bits of shadowed translations

[Sean Christopherson]

On Fri, Apr 22, 2022, David Matlack wrote:
> @@ -2820,7 +2861,10 @@ static int mmu_set_spte(struct kvm_vcpu *vcpu, struct kvm_memory_slot *slot,
>  
>  	if (!was_rmapped) {
>  		WARN_ON_ONCE(ret == RET_PF_SPURIOUS);
> -		rmap_add(vcpu, slot, sptep, gfn);
> +		rmap_add(vcpu, slot, sptep, gfn, pte_access);
> +	} else {
> +		/* Already rmapped but the pte_access bits may have changed. */
> +		kvm_mmu_page_set_access(sp, sptep - sp->spt, pte_access);
>  	}
>  
>  	return ret;

...

> diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
> index a8a755e1561d..97bf53b29b88 100644
> --- a/arch/x86/kvm/mmu/paging_tmpl.h
> +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> @@ -978,7 +978,8 @@ static gpa_t FNAME(gva_to_gpa)(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
>  }
>  
>  /*
> - * Using the cached information from sp->gfns is safe because:
> + * Using the information in sp->shadowed_translation (kvm_mmu_page_get_gfn()
> + * and kvm_mmu_page_get_access()) is safe because:
>   * - The spte has a reference to the struct page, so the pfn for a given gfn
>   *   can't change unless all sptes pointing to it are nuked first.
>   *
> @@ -1052,12 +1053,15 @@ static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
>  		if (sync_mmio_spte(vcpu, &sp->spt[i], gfn, pte_access))
>  			continue;
>  
> -		if (gfn != sp->gfns[i]) {
> +		if (gfn != kvm_mmu_page_get_gfn(sp, i)) {
>  			drop_spte(vcpu->kvm, &sp->spt[i]);
>  			flush = true;
>  			continue;
>  		}
>  
> +		if (pte_access != kvm_mmu_page_get_access(sp, i))

I think it makes sense to do this unconditionally, same as mmu_set_spte().  Or
make the mmu_set_spte() case conditional.  I don't have a strong preference either
way, but the two callers should be consistent with each other.

> +			kvm_mmu_page_set_access(sp, i, pte_access);
> +
>  		sptep = &sp->spt[i];
>  		spte = *sptep;
>  		host_writable = spte & shadow_host_writable_mask;
> -- 
> 2.36.0.rc2.479.g8af0fa9b8e-goog
>