Maybe a slight tweak to the shortlog? "Zap collapsible SPTEs at all levels in
the shadow MMU" left me wondering "when is KVM zapping at all levels?"
KVM: x86/mmu: Zap all possible levels in shadow MMU when collapsing SPTEs
On Fri, Apr 22, 2022, 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.
"partially" is really confusing. I think what you mean is that KVM can split a
1gb to a 2mb page, and not split all the way down to 4kb. But "partially" makes
it sound like KVM ends up with a huge SPTE that is half split or something. I
think you can just avoid that altogether and be more explicit:
i.e. a 1GiB pager is never split to just 2MiB, dirty logging always splits
down to 4KiB pages.
> However, this will stop being true once the shadow MMU participates in
> eager page splitting, which can in fact leave behind partially split
"partially" again. Maybe
which can in fact leave behind 2MiB pages after splitting 1GiB huge pages.
> 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.
>
> Reviewed-by: Peter Xu <peterx@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 ed65899d15a2..479c581e8a96 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -6098,18 +6098,25 @@ 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)
> +{
> + /*
> + * Note, use KVM_MAX_HUGEPAGE_LEVEL - 1 since there's no need to zap
> + * pages that are already mapped at the maximum possible level.
> + */
> + if (slot_handle_level(kvm, slot, kvm_mmu_zap_collapsible_spte,
> + PG_LEVEL_4K, KVM_MAX_HUGEPAGE_LEVEL - 1,
> + true))
> + 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);
> }
>
> --
> 2.36.0.rc2.479.g8af0fa9b8e-goog
>
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