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SubjectRe: [PATCH 3/4] KVM: x86/mmu: explicitly check nx_hugepage in disallowed_hugepage_adjust()
On Sun, Mar 20, 2022 at 5:26 PM Mingwei Zhang <mizhang@google.com> wrote:
>
> Add extra check to specify the case of nx hugepage and allow KVM to
> reconstruct large mapping after dirty logging is disabled. Existing code
> works only for nx hugepage but the condition is too general in that does
> not consider other usage case (such as dirty logging).

KVM calls kvm_mmu_zap_collapsible_sptes() when dirty logging is
disabled. Why is that not sufficient?

> Moreover, existing
> code assumes that a present PMD or PUD indicates that there exist 'smaller
> SPTEs' under the paging structure. This assumption may no be true if
> consider the zapping leafs only behavior in MMU.

Good point. Although, that code just got reverted. Maybe say something like:

This assumption may not be true in the future if KVM gains support
for zapping only leaf SPTEs.

>
> Missing the check causes KVM incorrectly regards the faulting page as a NX
> huge page and refuse to map it at desired level. And this leads to back
> performance in shadow mmu and potentiall TDP mmu.

s/potentiall/potentially/

>
> Fixes: b8e8c8303ff2 ("kvm: mmu: ITLB_MULTIHIT mitigation")
> Cc: stable@vger.kernel.org
>
> Reviewed-by: Ben Gardon <bgardon@google.com>
> Signed-off-by: Mingwei Zhang <mizhang@google.com>
> ---
> arch/x86/kvm/mmu/mmu.c | 14 ++++++++++++--
> 1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 5628d0ba637e..4d358c273f6c 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2919,6 +2919,16 @@ void disallowed_hugepage_adjust(struct kvm_page_fault *fault, u64 spte, int cur_
> cur_level == fault->goal_level &&
> is_shadow_present_pte(spte) &&
> !is_large_pte(spte)) {
> + struct kvm_mmu_page *sp;
> + u64 page_mask;
> + /*
> + * When nx hugepage flag is not set, there is no reason to
> + * go down to another level. This helps demand paging to
> + * generate large mappings.
> + */
> + sp = to_shadow_page(spte & PT64_BASE_ADDR_MASK);
> + if (!sp->lpage_disallowed)
> + return;
> /*
> * A small SPTE exists for this pfn, but FNAME(fetch)
> * and __direct_map would like to create a large PTE
> @@ -2926,8 +2936,8 @@ void disallowed_hugepage_adjust(struct kvm_page_fault *fault, u64 spte, int cur_
> * patching back for them into pfn the next 9 bits of
> * the address.
> */
> - u64 page_mask = KVM_PAGES_PER_HPAGE(cur_level) -
> - KVM_PAGES_PER_HPAGE(cur_level - 1);
> + page_mask = KVM_PAGES_PER_HPAGE(cur_level) -
> + KVM_PAGES_PER_HPAGE(cur_level - 1);
> fault->pfn |= fault->gfn & page_mask;
> fault->goal_level--;
> }
> --
> 2.35.1.894.gb6a874cedc-goog
>

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