[src/trunk]: src/sys/arch/xen/xen xencons(4): Reduce unnecessary membars.

[riastradh <riastradh%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/c079ec2a886f
branches:  trunk
changeset: 373716:c079ec2a886f
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Sat Feb 25 00:33:38 2023 +0000

description:
xencons(4): Reduce unnecessary membars.

- xencons_handler: After advancing in_cons, only need one xen_wmb
  before notifying the hypervisor that we're ready for more.

  (XXX Should this do xen_mb and re-check in_prod at that point, or
  does hypervisor_notify_via_evtchn obviate the need for this?)

- xenvonscn_getc: After reading in_prod, only need one xen_rmb before
  using the slots it is telling us are now ready.

diffstat:

 sys/arch/xen/xen/xencons.c |  7 +++----
 1 files changed, 3 insertions(+), 4 deletions(-)

diffs (37 lines):

diff -r 7b2910c01803 -r c079ec2a886f sys/arch/xen/xen/xencons.c
--- a/sys/arch/xen/xen/xencons.c        Sat Feb 25 00:33:27 2023 +0000
+++ b/sys/arch/xen/xen/xencons.c        Sat Feb 25 00:33:38 2023 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: xencons.c,v 1.51 2023/02/25 00:33:27 riastradh Exp $   */
+/*     $NetBSD: xencons.c,v 1.52 2023/02/25 00:33:38 riastradh Exp $   */
 
 /*
  * Copyright (c) 2006 Manuel Bouyer.
@@ -53,7 +53,7 @@
 
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: xencons.c,v 1.51 2023/02/25 00:33:27 riastradh Exp $");
+__KERNEL_RCSID(0, "$NetBSD: xencons.c,v 1.52 2023/02/25 00:33:38 riastradh Exp $");
 
 #include "opt_xen.h"
 
@@ -479,9 +479,9 @@
                        cons += len;
                        xen_wmb();
                        xencons_interface->in_cons = cons;
-                       xen_wmb();
                }
        }
+       xen_wmb();
        hypervisor_notify_via_evtchn(xen_start_info.console.domU.evtchn);
        splx(s);
        return 1;
@@ -575,7 +575,6 @@
 
        cons = xencons_interface->in_cons;
        prod = xencons_interface->in_prod;
-       xen_rmb();
        while (cons == prod) {
                HYPERVISOR_yield();
                prod = xencons_interface->in_prod;