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

[Mingwei Zhang <mizhang@google.com>]

On Mon, Mar 21, 2022, Ben Gardon wrote:
> 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). 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.
> >
> > 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.
> >
> > 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.
> > +                */
> 
> This comment is relevant to Google's internal demand paging scheme,
> but isn't really relevant to UFFD demand paging.
> Still, as demonstrated by the next commit, this is important for dirty
> loggin, so I'd suggest updating this comment to refer to that instead.
> 

Ah, leaking my true motivation :-) Definitely will update the comment.

> > +               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
> >