Re: [PATCH v2 18/26] KVM: x86/mmu: Zap collapsible SPTEs at all levels in the shadow MMU

David Matlack
2022-03-22T23:12:01Z

On Wed, Mar 16, 2022 at 1:49 AM Peter Xu <peterx@redhat.com> wrote:
>
> On Fri, Mar 11, 2022 at 12:25:20AM +0000, David Matlack wrote:
> > Currently KVM only zaps collapsible 4KiB SPTEs in the shadow MMU (i.e.
> > in the rmap). This is fine for now KVM never creates intermediate huge
> > pages during dirty logging, i.e. a 1GiB page is never partially split to
> > a 2MiB page.
> >
> > However, this will stop being true 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 necessary levels when zapping collapsible SPTEs.
> >
> > No functional change intended.
> >
> > Signed-off-by: David Matlack <dmatlack@google.com>
> > ---
> > arch/x86/kvm/mmu/mmu.c | 26 +++++++++++++++++++-------
> > 1 file changed, 19 insertions(+), 7 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 89a7a8d7a632..2032be3edd71 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -6142,18 +6142,30 @@ 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;
> > +
> > + /*
> > + * Note, use KVM_MAX_HUGEPAGE_LEVEL - 1 since there's no need to zap
> > + * pages that are already mapped at the maximum possible level.
> > + */
> > + flush = slot_handle_level(kvm, slot, kvm_mmu_zap_collapsible_spte,
> > + PG_LEVEL_4K, KVM_MAX_HUGEPAGE_LEVEL - 1,
> > + true);
> > +
> > + if (flush)
> > + kvm_arch_flush_remote_tlbs_memslot(kvm, slot);
> > +
> > +}
>
> Reviewed-by: Peter Xu <peterx@redhat.com>
>
> IMHO it looks cleaner to write it in the old way (drop the flush var).
> Maybe even unwrap the helper?

Unwrapping the helper and dropping the flush var makes the if
condition quite long and hard to read. But I think a compromise would
to have kvm_rmap_zap_collapsible_sptes() return flush and leave the
flush call in kvm_mmu_zap_collapsible_sptes().

>
> Thanks,
>
> --
> Peter Xu
>