[PATCH v2] kvm: mmu: Use fast PF path for access tracking of huge pages when possible

[PATCH v2] kvm: mmu: Use fast PF path for access tracking of huge pages when possible

[Junaid Shahid]

The fast page fault path bails out on write faults to huge pages in
order to accommodate dirty logging. This change adds a check to do that
only when dirty logging is actually enabled, so that access tracking for
huge pages can still use the fast path for write faults in the common
case.

Signed-off-by: Junaid Shahid <junaids@google.com>
Reviewed-by: Ben Gardon <bgardon@google.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
---
v2:
 - Removed a stale comment

 arch/x86/kvm/mmu/mmu.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 354d2ca92df4..04c00c34517e 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3191,17 +3191,17 @@ static int fast_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 			new_spte |= PT_WRITABLE_MASK;
 
 			/*
-			 * Do not fix write-permission on the large spte.  Since
-			 * we only dirty the first page into the dirty-bitmap in
+			 * Do not fix write-permission on the large spte when
+			 * dirty logging is enabled. Since we only dirty the
+			 * first page into the dirty-bitmap in
 			 * fast_pf_fix_direct_spte(), other pages are missed
 			 * if its slot has dirty logging enabled.
 			 *
 			 * Instead, we let the slow page fault path create a
 			 * normal spte to fix the access.
-			 *
-			 * See the comments in kvm_arch_commit_memory_region().
 			 */
-			if (sp->role.level > PG_LEVEL_4K)
+			if (sp->role.level > PG_LEVEL_4K &&
+			    kvm_slot_dirty_track_enabled(fault->slot))
 				break;
 		}
 
-- 
2.33.1.1089.g2158813163f-goog

Re: [PATCH v2] kvm: mmu: Use fast PF path for access tracking of huge pages when possible

[Paolo Bonzini]

On 11/4/21 01:33, Junaid Shahid wrote:
> The fast page fault path bails out on write faults to huge pages in
> order to accommodate dirty logging. This change adds a check to do that
> only when dirty logging is actually enabled, so that access tracking for
> huge pages can still use the fast path for write faults in the common
> case.
> 
> Signed-off-by: Junaid Shahid <junaids@google.com>
> Reviewed-by: Ben Gardon <bgardon@google.com>
> Reviewed-by: Sean Christopherson <seanjc@google.com>
> ---
> v2:
>   - Removed a stale comment
> 
>   arch/x86/kvm/mmu/mmu.c | 10 +++++-----
>   1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 354d2ca92df4..04c00c34517e 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3191,17 +3191,17 @@ static int fast_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>   			new_spte |= PT_WRITABLE_MASK;
>   
>   			/*
> -			 * Do not fix write-permission on the large spte.  Since
> -			 * we only dirty the first page into the dirty-bitmap in
> +			 * Do not fix write-permission on the large spte when
> +			 * dirty logging is enabled. Since we only dirty the
> +			 * first page into the dirty-bitmap in
>   			 * fast_pf_fix_direct_spte(), other pages are missed
>   			 * if its slot has dirty logging enabled.
>   			 *
>   			 * Instead, we let the slow page fault path create a
>   			 * normal spte to fix the access.
> -			 *
> -			 * See the comments in kvm_arch_commit_memory_region().
>   			 */
> -			if (sp->role.level > PG_LEVEL_4K)
> +			if (sp->role.level > PG_LEVEL_4K &&
> +			    kvm_slot_dirty_track_enabled(fault->slot))
>   				break;
>   		}
>   
> 

Queued, thanks.

Paolo