Re: [Xen-devel] [PATCH v2 33/34] xenbus: use virt_xxx barriers

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

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@xxxxxxxxxx>
> ---
>  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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