linux-kernel - Re: [PATCH v2 33/34] xenbus: use virt

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Message-ID: <20160104140939.GC6344@twins.programming.kicks-ass.net>
Date:	Mon, 4 Jan 2016 15:09:39 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
	linux-arch@...r.kernel.org,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	virtualization@...ts.linux-foundation.org,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	David Miller <davem@...emloft.net>, linux-ia64@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
	sparclinux@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-metag@...r.kernel.org, linux-mips@...ux-mips.org,
	x86@...nel.org, user-mode-linux-devel@...ts.sourceforge.net,
	adi-buildroot-devel@...ts.sourceforge.net,
	linux-sh@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
	xen-devel@...ts.xenproject.org,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>
Subject: Re: [PATCH v2 33/34] xenbus: use virt_xxx barriers

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@...hat.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.
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