Re: [PATCH 3/4] KVM: x86/mmu: explicitly check nx_hugepage in disallowed_hugepage_adjust()

[David Matlack]

On Sun, Mar 20, 2022 at 5:26 PM Mingwei Zhang <mizhang@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx
>
> Reviewed-by: Ben Gardon <bgardon@xxxxxxxxxx>
> Signed-off-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
> ---
>  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
>