[src/trunk]: src/sys/dev/pci Subtly adjust criteria for notification of Virti...

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

details:   https://anonhg.NetBSD.org/src/rev/39ef4f1ddbaa
branches:  trunk
changeset: 372901:39ef4f1ddbaa
user:      jakllsch <jakllsch%NetBSD.org@localhost>
date:      Tue Jan 03 19:33:31 2023 +0000

description:
Subtly adjust criteria for notification of Virtio devices in
VIRTIO_F_RING_EVENT_IDX mode.

This fixes stalls in the vioif(4) transmit path that would happen sooner
or later, depending on interface transmit utilization, and were
particularly noticable with a NetBSD guest transmitting to its x86 KVM
host at multi-gigabit speeds.

diffstat:

 sys/dev/pci/virtio.c |  6 +++---
 1 files changed, 3 insertions(+), 3 deletions(-)

diffs (27 lines):

diff -r c44a28e61ffd -r 39ef4f1ddbaa sys/dev/pci/virtio.c
--- a/sys/dev/pci/virtio.c      Tue Jan 03 18:30:33 2023 +0000
+++ b/sys/dev/pci/virtio.c      Tue Jan 03 19:33:31 2023 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: virtio.c,v 1.64 2022/12/30 21:38:13 jakllsch Exp $     */
+/*     $NetBSD: virtio.c,v 1.65 2023/01/03 19:33:31 jakllsch Exp $     */
 
 /*
  * Copyright (c) 2020 The NetBSD Foundation, Inc.
@@ -28,7 +28,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: virtio.c,v 1.64 2022/12/30 21:38:13 jakllsch Exp $");
+__KERNEL_RCSID(0, "$NetBSD: virtio.c,v 1.65 2023/01/03 19:33:31 jakllsch Exp $");
 
 #include <sys/param.h>
 #include <sys/systm.h>
@@ -1148,7 +1148,7 @@
                uint16_t o, n, t;
                uint16_t flags;
 
-               o = virtio_rw16(sc, vq->vq_avail->idx);
+               o = virtio_rw16(sc, vq->vq_avail->idx) - 1;
                n = vq->vq_avail_idx;
 
                /*