Re: [PATCH v6 7/8] KVM: x86/mmu: explicitly check nx_hugepage in disallowed_hugepage_adjust()

[Yan Zhao]

Reviewed-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>

On Wed, Oct 19, 2022 at 04:56:17PM +0000, Sean Christopherson wrote:
> From: Mingwei Zhang <mizhang@xxxxxxxxxx>
> 
> Explicitly check if a NX huge page is disallowed when determining if a
> page fault needs to be forced to use a smaller sized page.  KVM currently
> assumes that the NX huge page mitigation is the only scenario where KVM
> will force a shadow page instead of a huge page, and so unnecessarily
> keeps an existing shadow page instead of replacing it with a huge page.
> 
> Any scenario that causes KVM to zap leaf SPTEs may result in having a SP
> that can be made huge without violating the NX huge page mitigation.
> E.g. prior to commit 5ba7c4c6d1c7 ("KVM: x86/MMU: Zap non-leaf SPTEs when
> disabling dirty logging"), KVM would keep shadow pages after disabling
> dirty logging due to a live migration being canceled, resulting in
> degraded performance due to running with 4kb pages instead of huge pages.
> 
> Although the dirty logging case is "fixed", that fix is coincidental,
> i.e. is an implementation detail, and there are other scenarios where KVM
> will zap leaf SPTEs.  E.g. zapping leaf SPTEs in response to a host page
> migration (mmu_notifier invalidation) to create a huge page would yield a
> similar result; KVM would see the shadow-present non-leaf SPTE and assume
> a huge page is disallowed.
> 
> Fixes: b8e8c8303ff2 ("kvm: mmu: ITLB_MULTIHIT mitigation")
> Reviewed-by: Ben Gardon <bgardon@xxxxxxxxxx>
> Reviewed-by: David Matlack <dmatlack@xxxxxxxxxx>
> Signed-off-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
> [sean: use spte_to_child_sp(), massage changelog, fold into if-statement]
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/mmu/mmu.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index f4f1b1591a02..14674c9e10f7 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3111,7 +3111,8 @@ void disallowed_hugepage_adjust(struct kvm_page_fault *fault, u64 spte, int cur_
>  	if (cur_level > PG_LEVEL_4K &&
>  	    cur_level == fault->goal_level &&
>  	    is_shadow_present_pte(spte) &&
> -	    !is_large_pte(spte)) {
> +	    !is_large_pte(spte) &&
> +	    spte_to_child_sp(spte)->nx_huge_page_disallowed) {
>  		/*
>  		 * A small SPTE exists for this pfn, but FNAME(fetch)
>  		 * and __direct_map would like to create a large PTE
> -- 
> 2.38.0.413.g74048e4d9e-goog
>