Re: [PATCH 01/23] KVM: x86/mmu: Optimize MMU page cache lookup for all direct SPs

[Sean Christopherson <seanjc@google.com>]

On Thu, Feb 03, 2022, David Matlack wrote:
> Commit fb58a9c345f6 ("KVM: x86/mmu: Optimize MMU page cache lookup for
> fully direct MMUs") skipped the unsync checks and write flood clearing
> for full direct MMUs. We can extend this further and skip the checks for
> all direct shadow pages. Direct shadow pages are never marked unsynced
> or have a non-zero write-flooding count.
> 
> No functional change intended.
> 
> Signed-off-by: David Matlack <dmatlack@google.com>
> ---

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

>  arch/x86/kvm/mmu/mmu.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 296f8723f9ae..6ca38277f2ab 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2052,7 +2052,6 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
>  					     int direct,
>  					     unsigned int access)
>  {
> -	bool direct_mmu = vcpu->arch.mmu->direct_map;
>  	union kvm_mmu_page_role role;
>  	struct hlist_head *sp_list;
>  	unsigned quadrant;
> @@ -2093,7 +2092,8 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
>  			continue;
>  		}
>  
> -		if (direct_mmu)
> +		/* unsync and write-flooding only apply to indirect SPs. */
> +		if (sp->role.direct)

Because I spent waaaay too much time over-analyzing this... checking sp->role.direct
actually generates better code than check @direct.  Because of regsiter pressure,
direct has to get shoved onto the stack and then pulled back off.  Not that it
matters, at all, because this code runs exactly once...

>  			goto trace_get_page;
>  
>  		if (sp->unsync) {
> -- 
> 2.35.0.rc2.247.g8bbb082509-goog
>