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

[Paolo Bonzini]

----- Original Message -----
> From: "Wanpeng Li" <kernellwp@xxxxxxxxx>
> To: linux-kernel@xxxxxxxxxxxxxxx, kvm@xxxxxxxxxxxxxxx
> Cc: "Paolo Bonzini" <pbonzini@xxxxxxxxxx>, "Radim KrÄmÃÅ" <rkrcmar@xxxxxxxxxx>, "Wanpeng Li" <wanpeng.li@xxxxxxxxxxx>
> 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@xxxxxxxxxxx>
> 
> 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@xxxxxxxxxx>
> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> ---
>  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@xxxxxxxxxx>