linux-kernel - Re: [PATCH v3 1/3] KVM: x86/mmu: Remove redundant spte present check in mmu_set

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Message-ID: <YQhxv3XO931lk7ny@google.com>
Date:   Mon, 2 Aug 2021 22:29:19 +0000
From:   David Matlack <dmatlack@...gle.com>
To:     Mingwei Zhang <mizhang@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.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 v3 1/3] KVM: x86/mmu: Remove redundant spte present check
 in mmu_set_spte

On Fri, Jul 30, 2021 at 03:59:37PM -0700, 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.
> 
> No functional change intended.
> 
> Suggested-by: Ben Gardon <bgardon@...gle.com>
> 
> Reviewed-by: Ben Gardon <bgardon@...gle.com>
> Reviewed-by: Sean Christopherson <seanjc@...gle.com>
> Signed-off-by: Mingwei Zhang <mizhang@...gle.com>
> ---

Reviewed-by: David Matlack <dmatlack@...gle.com>

>  arch/x86/kvm/mmu/mmu.c | 14 ++++++--------
>  1 file changed, 6 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index b888385d1933..442cc554ebd6 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2690,15 +2690,13 @@ static int mmu_set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
>  
>  	pgprintk("%s: setting spte %llx\n", __func__, *sptep);
>  	trace_kvm_mmu_set_spte(level, gfn, 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) {
> +		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;
> -- 
> 2.32.0.554.ge1b32706d8-goog
>