On Mon, Feb 28, 2022 at 12:40 PM Ben Gardon <bgardon@xxxxxxxxxx> wrote:
>
> On Wed, Feb 2, 2022 at 5:02 PM David Matlack <dmatlack@xxxxxxxxxx> wrote:
> >
> > Currently KVM only zaps collapsible 4KiB SPTEs in the shadow MMU (i.e.
> > in the rmap). This leads to correct behavior because KVM never creates
> > intermediate huge pages during dirty logging. For example, a 1GiB page
> > is never partially split to a 2MiB page.
> >
> > However this behavior will stop being correct once the shadow MMU
> > participates in eager page splitting, which can in fact leave behind
> > partially split huge pages. In preparation for that change, change the
> > shadow MMU to iterate over all levels when zapping collapsible SPTEs.
> >
> > No functional change intended.
> >
>
> Reviewed-by: Ben Gardon <bgardon@xxxxxxxxxx>
>
> > Signed-off-by: David Matlack <dmatlack@xxxxxxxxxx>
> > ---
> > arch/x86/kvm/mmu/mmu.c | 21 ++++++++++++++-------
> > 1 file changed, 14 insertions(+), 7 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index e2306a39526a..99ad7cc8683f 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -6038,18 +6038,25 @@ static bool kvm_mmu_zap_collapsible_spte(struct kvm *kvm,
> > return need_tlb_flush;
> > }
> >
> > +static void kvm_rmap_zap_collapsible_sptes(struct kvm *kvm,
> > + const struct kvm_memory_slot *slot)
> > +{
> > + bool flush;
> > +
> > + flush = slot_handle_level(kvm, slot, kvm_mmu_zap_collapsible_spte,
> > + PG_LEVEL_4K, KVM_MAX_HUGEPAGE_LEVEL, true);
>
> The max level here only needs to be 2M since 1G page wouldn't be
> split. I think the upper limit can be lowered to
> KVM_MAX_HUGEPAGE_LEVEL - 1.
> Not a significant performance difference though.
Good point. There's no reason to look at huge pages that are already
mapped at the maximum possible level.
>
> > +
> > + if (flush)
> > + kvm_arch_flush_remote_tlbs_memslot(kvm, slot);
> > +
> > +}
> > +
> > void kvm_mmu_zap_collapsible_sptes(struct kvm *kvm,
> > const struct kvm_memory_slot *slot)
> > {
> > if (kvm_memslots_have_rmaps(kvm)) {
> > write_lock(&kvm->mmu_lock);
> > - /*
> > - * Zap only 4k SPTEs since the legacy MMU only supports dirty
> > - * logging at a 4k granularity and never creates collapsible
> > - * 2m SPTEs during dirty logging.
> > - */
> > - if (slot_handle_level_4k(kvm, slot, kvm_mmu_zap_collapsible_spte, true))
> > - kvm_arch_flush_remote_tlbs_memslot(kvm, slot);
> > + kvm_rmap_zap_collapsible_sptes(kvm, slot);
> > write_unlock(&kvm->mmu_lock);
> > }
> >
> > --
> > 2.35.0.rc2.247.g8bbb082509-goog
> >
