linux-kernel - Re: [PATCH] KVM/MMU: Combine flushing remote tlb in mmu_set

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Date:   Tue, 24 Jul 2018 16:35:21 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Tianyu Lan <Tianyu.Lan@...rosoft.com>
Cc:     "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "hpa@...or.com" <hpa@...or.com>, "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
        Junaid Shahid <junaids@...gle.com>
Subject: Re: [PATCH] KVM/MMU: Combine flushing remote tlb in mmu_set_spte()

On 24/07/2018 10:17, Tianyu Lan wrote:
> mmu_set_spte() flushes remote tlbs for drop_parent_pte/drop_spte()
> and set_spte() separately. This may introduce redundant flush. This
> patch is to combine these flushes and check flush request after
> calling set_spte().
> 
> Signed-off-by: Lan Tianyu <Tianyu.Lan@...rosoft.com>

Looks good, but I'd like a second opinion.  Guangrong, Junaid, can you
review this?

Thanks,

Paolo

> ---
>  arch/x86/kvm/mmu.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 22a7984..8f21632 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -2901,6 +2901,7 @@ static int mmu_set_spte(struct kvm_vcpu *vcpu, u64 *sptep, unsigned pte_access,
>  	int rmap_count;
>  	int set_spte_ret;
>  	int ret = RET_PF_RETRY;
> +	bool flush = false;
>  
>  	pgprintk("%s: spte %llx write_fault %d gfn %llx\n", __func__,
>  		 *sptep, write_fault, gfn);
> @@ -2917,12 +2918,12 @@ static int mmu_set_spte(struct kvm_vcpu *vcpu, u64 *sptep, unsigned pte_access,
>  
>  			child = page_header(pte & PT64_BASE_ADDR_MASK);
>  			drop_parent_pte(child, sptep);
> -			kvm_flush_remote_tlbs(vcpu->kvm);
> +			flush = true;
>  		} else if (pfn != spte_to_pfn(*sptep)) {
>  			pgprintk("hfn old %llx new %llx\n",
>  				 spte_to_pfn(*sptep), pfn);
>  			drop_spte(vcpu->kvm, sptep);
> -			kvm_flush_remote_tlbs(vcpu->kvm);
> +			flush = true;
>  		} else
>  			was_rmapped = 1;
>  	}
> @@ -2934,7 +2935,7 @@ static int mmu_set_spte(struct kvm_vcpu *vcpu, u64 *sptep, unsigned pte_access,
>  			ret = RET_PF_EMULATE;
>  		kvm_make_request(KVM_REQ_TLB_FLUSH, vcpu);
>  	}
> -	if (set_spte_ret & SET_SPTE_NEED_REMOTE_TLB_FLUSH)
> +	if (set_spte_ret & SET_SPTE_NEED_REMOTE_TLB_FLUSH || flush)
>  		kvm_flush_remote_tlbs(vcpu->kvm);
>  
>  	if (unlikely(is_mmio_spte(*sptep)))
>