Drivers: hv: vmbus: Support kexec on ws2012 r2 and above

BugLink: http://bugs.launchpad.net/bugs/1558720

WS2012 R2 and above hosts can support kexec in that thay can support
reconnecting to the host (as would be needed in the kexec path)
on any CPU. Enable this. Pre ws2012 r2 hosts don't have this ability
and consequently cannot support kexec.
Signed-off-by: Alex Ng <alexng@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from linux-next commit 72686447)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

drivers/hv/connection.c

@@ -88,8 +88,16 @@ static int vmbus_negotiate_version(struct vmbus_channel_msginfo *msginfo,
 	 * This has been the behavior pre-win8. This is not
 	 * perf issue and having all channel messages delivered on CPU 0
 	 * would be ok.
+	 * For post win8 hosts, we support receiving channel messagges on
+	 * all the CPUs. This is needed for kexec to work correctly where
+	 * the CPU attempting to connect may not be CPU 0.
 	 */
-	msg->target_vcpu = 0;
+	if (version >= VERSION_WIN8_1) {
+		msg->target_vcpu = hv_context.vp_index[get_cpu()];
+		put_cpu();
+	} else {
+		msg->target_vcpu = 0;
+	}
 
 	/*
 	 * Add to list before we send the request since we may