Subject: Re: [PATCH] KVM: Remove redundant smp_mb() in the
 kvm_mmu_commit_zap_page()
To: Lan Tianyu <tianyu.lan@intel.com>, gleb@kernel.org, tglx@linutronix.de,
        mingo@redhat.com, hpa@zytor.com, x86@kernel.org
References: <1457055312-27067-1-git-send-email-tianyu.lan@intel.com>
Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        Xiao Guangrong <guangrong.xiao@linux.intel.com>
From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <56D9417E.4000206@redhat.com>
Date: Fri, 4 Mar 2016 09:04:14 +0100
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On 04/03/2016 02:35, Lan Tianyu wrote:
> The following kvm_flush_remote_tlbs() will call smp_mb() inside and so
> remove smp_mb() here.
> 
> Signed-off-by: Lan Tianyu <tianyu.lan@intel.com>
> ---
>  arch/x86/kvm/mmu.c | 6 ------
>  1 file changed, 6 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index a54ecd9..6315416 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -2383,12 +2383,6 @@ static void kvm_mmu_commit_zap_page(struct kvm *kvm,
>  		return;
>  
>  	/*
> -	 * wmb: make sure everyone sees our modifications to the page tables
> -	 * rmb: make sure we see changes to vcpu->mode
> -	 */
> -	smp_mb();
> -
> -	/*
>  	 * Wait for all vcpus to exit guest mode and/or lockless shadow
>  	 * page table walks.
>  	 */
> 

kvm_flush_remote_tlbs loads kvm->tlbs_dirty before issuing the memory
barrier.  I think it's okay if the load is done earlier, but I'd like
Guangrong to take a look.

In any case, this patch is not going to be included in 4.6.  I'll queue
it directly for 4.7.

Paolo

Looks good, but I'd like Xiao to take a look.