Re: NetBSD 10.1 VPS became unresponsive

[Christof Meerwald <cmeerw%cmeerw.org@localhost>]

On Mon, Aug 25, 2025 at 05:46:07AM -0000, Michael van Elst wrote:
> cmeerw%cmeerw.org@localhost (Christof Meerwald) writes:
> 
> >So it does look like some kind of race condition, also I thought that
> >should be handled by qemu by first setting the vq->vq_used->flags to 0
> >and then checking vq->vq_used->idx again before relying on
> >notifications being sent.
> 
> Maybe a memory ordering issue then ?

Yes, probably - I have added an mfence and not seen any issue since
then (more than 2 days now, with my sync loop running in the
background). BTW, this is on a AMD Ryzen 9 9950X 16-Core Processor
(with 2 CPUs assigned to the VPS).


Index: dev/pci/virtio.c
===================================================================
RCS file: /cvsroot/src/sys/dev/pci/virtio.c,v
retrieving revision 1.63.2.6
diff -u -r1.63.2.6 virtio.c
--- dev/pci/virtio.c	2 Oct 2024 18:20:48 -0000	1.63.2.6
+++ dev/pci/virtio.c	28 Aug 2025 21:42:47 -0000
@@ -38,6 +38,7 @@
 #include <sys/device.h>
 #include <sys/kmem.h>
 #include <sys/module.h>
+#include <x86/cpufunc.h>
 
 #define VIRTIO_PRIVATE
 
@@ -1244,6 +1248,7 @@
 		vq->vq_avail->idx = virtio_rw16(sc, vq->vq_avail_idx);
 		vq_sync_aring_header(sc, vq, BUS_DMASYNC_PREWRITE);
 		vq->vq_queued++;
+		x86_mfence();
 
 		if (sc->sc_active_features & VIRTIO_F_RING_EVENT_IDX) {
 			vq_sync_uring_avail(sc, vq, BUS_DMASYNC_POSTREAD);


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