Re: [PATCH v2 33/34] xenbus: use virt_xxx barriers
From: Peter Zijlstra <peterz@infradead.org>
Date: 2016-01-04 14:10:00
Also in:
linux-arch, linux-arm-kernel, linux-mips, linux-s390, linux-sh, linux-um, lkml, sparclinux
On Thu, Dec 31, 2015 at 09:10:01PM +0200, Michael S. Tsirkin wrote:
quoted hunk ↗ jump to hunk
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.