diff mbox series

[v4,17/20] KVM: x86/mmu: Zap collapsible SPTEs at all levels in the shadow MMU

Message ID 20220422210546.458943-18-dmatlack@google.com (mailing list archive)
State New, archived
Headers show
Series KVM: Extend Eager Page Splitting to the shadow MMU | expand

Commit Message

David Matlack April 22, 2022, 9:05 p.m. UTC 
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.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: David Matlack <dmatlack@google.com>
---
 arch/x86/kvm/mmu/mmu.c | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

Comments

Sean Christopherson May 9, 2022, 4:31 p.m. UTC | #1 
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@redhat.com>
> Signed-off-by: David Matlack <dmatlack@google.com>
> ---
>  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
>
David Matlack May 9, 2022, 9:34 p.m. UTC | #2 
On Mon, May 9, 2022 at 9:31 AM Sean Christopherson <seanjc@google.com> wrote:
>
> 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.

Looks good, I'll incorporate these edits into v5.

>
> > 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@redhat.com>
> > Signed-off-by: David Matlack <dmatlack@google.com>
> > ---
> >  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
> >
diff mbox series

Patch

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);
 	}