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

[Sean Christopherson <seanjc@google.com>]

On Mon, Nov 01, 2021, 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>

One nit, otherwise

Reviewed-by: Sean Christopherson <seanjc@google.com>

> ---
>  arch/x86/kvm/mmu/mmu.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 354d2ca92df4..5df9181c5082 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3191,8 +3191,9 @@ 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.
>  			 *
> @@ -3201,7 +3202,8 @@ static int fast_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>  			 *
>  			 * See the comments in kvm_arch_commit_memory_region().

This part is slightly stale as kvm_mmu_slot_apply_flags() now has the comments.
Maybe just drop it entirely?  The comments there don't do a whole lot to make this
code more understandable.

>  			 */
> -			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
>