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.
> +
> + 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
>
