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

[Mingwei Zhang]

On Mon, Mar 21, 2022, David Matlack wrote:
> On Mon, Mar 21, 2022 at 3:00 PM David Matlack <dmatlack@xxxxxxxxxx> wrote:
> >
> > 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?
> 
> Ahh I see, kvm_mmu_zap_collapsible_sptes() only zaps the leaf SPTEs.
> Could you add a blurb about this in the commit message for future
> reference?
> 

will do.

> >
> > > 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.
> 
> Nevermind, support for zapping leaf SPTEs already exists for zapping
> collapsible 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/

Thanks for that.
> >
> > >
> > > 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
> > >