linux-kernel - Re: [PATCH 1/2] kvm: mmu/x86: Remove redundant spte present check in mmu_set

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Date:   Wed, 21 Jul 2021 16:27:13 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Mingwei Zhang <mizhang@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ben Gardon <bgardon@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>
Subject: Re: [PATCH 1/2] kvm: mmu/x86: Remove redundant spte present check in
 mmu_set_spte

On Tue, Jul 20, 2021, Mingwei Zhang wrote:
> Drop an unnecessary is_shadow_present_pte() check when updating the rmaps
> after installing a non-MMIO SPTE.  set_spte() is used only to create
> shadow-present SPTEs, e.g. MMIO SPTEs are handled early on, mmu_set_spte()
> runs with mmu_lock held for write, i.e. the SPTE can't be zapped between
> writing the SPTE and updating the rmaps.
> 
> Opportunistically combine the "new SPTE" logic for large pages and rmaps.

Heh, except you forgot to actually do this in the code.

> No functional change intended.
> 
> Suggested-by: Ben Gardon <bgardon@...gle.com>
> Signed-off-by: Mingwei Zhang <mizhang@...gle.com>
> ---
>  arch/x86/kvm/mmu/mmu.c | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index b888385d1933..c45ddd2c964f 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2693,12 +2693,10 @@ static int mmu_set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
>  	if (!was_rmapped && is_large_pte(*sptep))
>  		++vcpu->kvm->stat.lpages;
>  
> -	if (is_shadow_present_pte(*sptep)) {
> -		if (!was_rmapped) {
> -			rmap_count = rmap_add(vcpu, sptep, gfn);
> -			if (rmap_count > RMAP_RECYCLE_THRESHOLD)
> -				rmap_recycle(vcpu, sptep, gfn);
> -		}
> +	if (!was_rmapped) {

As above, this should be:

	if (!was_rmapped) {
		if (is_large_pte(*sptep))
			++vcpu->kvm->stat.lpages;

> +		rmap_count = rmap_add(vcpu, sptep, gfn);
> +		if (rmap_count > RMAP_RECYCLE_THRESHOLD)
> +			rmap_recycle(vcpu, sptep, gfn);
>  	}
>  
>  	return ret;
> -- 
> 2.32.0.402.g57bb445576-goog
>