linux-kernel - Re: [PATCH] KVM: Remove redundant smp_mb() in the kvm_mmu_commit_zap

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Date:	Fri, 4 Mar 2016 08:21:12 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Lan Tianyu <tianyu.lan@...el.com>
cc:	gleb@...nel.org, pbonzini@...hat.com, mingo@...hat.com,
	hpa@...or.com, x86@...nel.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: Remove redundant smp_mb() in the
 kvm_mmu_commit_zap_page()

On Fri, 4 Mar 2016, 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@...el.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

You want to leave the comment explaining the memory barriers and tell that
kvm_flush_remote_tlbs() contains the smp_mb().

> -	 */
> -	smp_mb();
> -
> -	/*
>  	 * Wait for all vcpus to exit guest mode and/or lockless shadow
>  	 * page table walks.
>  	 */
> -- 
> 1.8.4.rc0.1.g8f6a3e5.dirty
> 
>