From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1751676AbcADOKA (ORCPT <rfc822;w@1wt.eu>);
	Mon, 4 Jan 2016 09:10:00 -0500
Received: from bombadil.infradead.org ([198.137.202.9]:33478 "EHLO
	bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1750889AbcADOJ4 (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Mon, 4 Jan 2016 09:09:56 -0500
Date: Mon, 4 Jan 2016 15:09:39 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: linux-kernel@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
        linux-arch@vger.kernel.org, Andrew Cooper <andrew.cooper3@citrix.com>,
        virtualization@lists.linux-foundation.org,
        Stefano Stabellini <stefano.stabellini@eu.citrix.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>,
        "H. Peter Anvin" <hpa@zytor.com>, David Miller <davem@davemloft.net>,
        linux-ia64@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        linux-s390@vger.kernel.org, sparclinux@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-metag@vger.kernel.org,
        linux-mips@linux-mips.org, x86@kernel.org,
        user-mode-linux-devel@lists.sourceforge.net,
        adi-buildroot-devel@lists.sourceforge.net, linux-sh@vger.kernel.org,
        linux-xtensa@linux-xtensa.org, xen-devel@lists.xenproject.org,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        David Vrabel <david.vrabel@citrix.com>
Subject: Re: [PATCH v2 33/34] xenbus: use virt_xxx barriers
Message-ID: <20160104140939.GC6344@twins.programming.kicks-ass.net>
References: <1451572003-2440-1-git-send-email-mst@redhat.com>
 <1451572003-2440-34-git-send-email-mst@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1451572003-2440-34-git-send-email-mst@redhat.com>
User-Agent: Mutt/1.5.21 (2012-12-30)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Thu, Dec 31, 2015 at 09:10:01PM +0200, Michael S. Tsirkin wrote:
> drivers/xen/xenbus/xenbus_comms.c uses
> full memory barriers to communicate with the other side.
> 
> For guests compiled with CONFIG_SMP, smp_wmb and smp_mb
> would be sufficient, so mb() and wmb() here are only needed if
> a non-SMP guest runs on an SMP host.
> 
> Switch to virt_xxx barriers which serve this exact purpose.
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>  drivers/xen/xenbus/xenbus_comms.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/xen/xenbus/xenbus_comms.c b/drivers/xen/xenbus/xenbus_comms.c
> index fdb0f33..ecdecce 100644
> --- a/drivers/xen/xenbus/xenbus_comms.c
> +++ b/drivers/xen/xenbus/xenbus_comms.c
> @@ -123,14 +123,14 @@ int xb_write(const void *data, unsigned len)
>  			avail = len;
>  
>  		/* Must write data /after/ reading the consumer index. */
> -		mb();
> +		virt_mb();
>  

So its possible to remove this barrier entirely, see the "CONTROL
DEPENDNCIES" chunk of memory-barrier.txt.

But do that in a separate patch series and only if you really really
need the performance.