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Re: [Qemu-devel] [PULL 12/23] virtio: use VRingMemoryRegionCaches for av

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PULL 12/23] virtio: use VRingMemoryRegionCaches for avail and used rings
Date: Wed, 22 Feb 2017 10:03:56 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.0

On 21/02/2017 18:54, Laszlo Ersek wrote:
> Actually, QEMU segfaults. From the dmesg:
> [Tue Feb 21 18:47:28 2017] CPU 0/KVM[8298]: segfault at 48 ip
> 00007fcb5dd02105 sp 00007fcb49efc270 error 4 in
> qemu-system-x86_64[7fcb5dae3000+905000]
> Complete backtrace below. (Thread 11 seems to be the one
> segfaulting.)

Indeed.  It's the virtio-blk device that is segfaulting, please try
this one liner (haven't reproduced it, but it seems obvious with the
backtrace and some sleep).

commit c1aa478c7181c543606ca81404c59e126d66213d
Author: Paolo Bonzini <address@hidden>
Date:   Wed Feb 22 10:02:37 2017 +0100

    virtio: check for vring setup in virtio_queue_empty
    If the vring has not been set up, there is nothing in the virtqueue.
    virtio_queue_host_notifier_aio_poll calls virtio_queue_empty even in
    this case; we have to filter it out just like virtio_queue_notify_aio_vq.
    Reported-by: Gerd Hoffmann <address@hidden>
    Signed-off-by: Paolo Bonzini <address@hidden>

diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 23483c7..e487e36 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -2291,7 +2291,7 @@ static bool virtio_queue_host_notifier_aio_poll(void 
     VirtQueue *vq = container_of(n, VirtQueue, host_notifier);
     bool progress;
-    if (virtio_queue_empty(vq)) {
+    if (!vq->vring.desc || virtio_queue_empty(vq)) {
         return false;

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