linux-kernel - Re: [PATCH] x86/kvm: virt_xxx memory barriers instead of mandatory barriers

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Date:   Tue, 11 Apr 2017 10:20:16 -0400 (EDT)
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH] x86/kvm: virt_xxx memory barriers instead of mandatory
 barriers



----- Original Message -----
> From: "Wanpeng Li" <kernellwp@...il.com>
> To: linux-kernel@...r.kernel.org, kvm@...r.kernel.org
> Cc: "Paolo Bonzini" <pbonzini@...hat.com>, "Radim Krčmář" <rkrcmar@...hat.com>, "Wanpeng Li" <wanpeng.li@...mail.com>
> Sent: Tuesday, April 11, 2017 5:49:21 PM
> Subject: [PATCH] x86/kvm: virt_xxx memory barriers instead of mandatory barriers
> 
> From: Wanpeng Li <wanpeng.li@...mail.com>
> 
> virt_xxx memory barriers are implemented trivially using the low-level
> __smp_xxx macros, __smp_xxx is equal to a compiler barrier for strong
> TSO memory model, however, mandatory barriers will unconditional add
> memory barriers, this patch replaces the rmb() in kvm_steal_clock() by
> virt_rmb().
> 
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
>  arch/x86/kernel/kvm.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 14f65a5..da5c097 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -396,9 +396,9 @@ static u64 kvm_steal_clock(int cpu)
>  	src = &per_cpu(steal_time, cpu);
>  	do {
>  		version = src->version;
> -		rmb();
> +		virt_rmb();
>  		steal = src->steal;
> -		rmb();
> +		virt_rmb();
>  	} while ((version & 1) || (version != src->version));
>  
>  	return steal;
> --
> 2.7.4

Reviewed-by: Paolo Bonzini <pbonzini@...hat.com>