[ovs-dev] [PATCH 1/3] lib/ovs-atomic-i586: Faster 64-bit atomics on 32-bit builds with SSE.

[Ben Pfaff]

On Wed, Sep 24, 2014 at 11:24:00AM -0700, Jarno Rajahalme wrote:
> Aligned 64-bit memory accesses in i586 are atomic.  By using an SSE
> register we can make such memory accesses in one instruction without
> bus-locking.  Need to compile with -msse to enable this feature.
> 
> Signed-off-by: Jarno Rajahalme <jrajahalme at nicira.com>

I guess that ovs-atomic-i586 must be aimed at older versions of
XenServer, which always run on 64-bit capable processors but in 32-bit
mode.  That means that we can always build with -msse for XenServer.
Should we patch xenserver/openvswitch-xen.spec to do that? 

The non-SSE code in atomic_read_8__() is very clever.  I am not sure
that I would have thought of using the existing value in EBX:ECX as
the value to write as well.  It works around the PIC issue very well,
without needing any extra code.

I am not sure why the asm statements for reading atomic variables are
volatile.  I don't think they have any side effects.

Acked-by: Ben Pfaff <blp at nicira.com>