On Mon, Feb 18, 2019 at 07:21:25AM +0000, Fernando Casas Schössow wrote: > It took a few days but last night the problem was reproduced. > This is the information from the log: > > vdev 0x55f261d940f0 ("virtio-blk") > vq 0x55f261d9ee40 (idx 0) > inuse 128 vring.num 128 > old_shadow_avail_idx 58874 last_avail_idx 58625 avail_idx 58874 > avail 0x3d87a800 avail_idx (cache bypassed) 58625
Hi Paolo, Are you aware of any recent MemoryRegionCache issues? The avail_idx value 58874 was read via the cache while a non-cached read produces 58625! I suspect that 58625 is correct since the vring is already full and the driver wouldn't bump avail_idx any further until requests complete. Fernando also hits this issue with virtio-scsi so it's not a virtio_blk.ko driver bug or a virtio-blk device emulation issue. A QEMU core dump is available for debugging. Here is the patch that produced this debug output: diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c index a1ff647a66..28d89fcbcb 100644 --- a/hw/virtio/virtio.c +++ b/hw/virtio/virtio.c @@ -866,6 +866,7 @@ void *virtqueue_pop(VirtQueue *vq, size_t sz) return NULL; } rcu_read_lock(); + uint16_t old_shadow_avail_idx = vq->shadow_avail_idx; if (virtio_queue_empty_rcu(vq)) { goto done; } @@ -879,6 +880,12 @@ void *virtqueue_pop(VirtQueue *vq, size_t sz) max = vq->vring.num; if (vq->inuse >= vq->vring.num) { + fprintf(stderr, "vdev %p (\"%s\")\n", vdev, vdev->name); + fprintf(stderr, "vq %p (idx %u)\n", vq, (unsigned int)(vq - vdev->vq)); + fprintf(stderr, "inuse %u vring.num %u\n", vq->inuse, vq->vring.num); + fprintf(stderr, "old_shadow_avail_idx %u last_avail_idx %u avail_idx %u\n", old_shadow_avail_idx, vq->last_avail_idx, vq->shadow_avail_idx); + fprintf(stderr, "avail %#" HWADDR_PRIx " avail_idx (cache bypassed) %u\n", vq->vring.avail, virtio_lduw_phys(vdev, vq->vring.avail + offsetof(VRingAvail, idx))); + fprintf(stderr, "used_idx %u\n", vq->used_idx); + abort(); /* <--- core dump! */ virtio_error(vdev, "Virtqueue size exceeded"); goto done; } Stefan > used_idx 58497 > 2019-02-18 03:20:08.605+0000: shutting down, reason=crashed > > The dump file, including guest memory, was generated successfully (after gzip > the file is around 492MB). > I switched the guest now to virtio-scsi to get the information and dump with > this setup as well. > > How should we proceed? > > Thanks. > > On lun, feb 11, 2019 at 4:17 AM, Stefan Hajnoczi <stefa...@gmail.com> wrote: > Thanks for collecting the data! The fact that both virtio-blk and virtio-scsi > failed suggests it's not a virtqueue element leak in the virtio-blk or > virtio-scsi device emulation code. The hung task error messages from inside > the guest are a consequence of QEMU hitting the "Virtqueue size exceeded" > error. QEMU refuses to process further requests after the error, causing > tasks inside the guest to get stuck on I/O. I don't have a good theory > regarding the root cause. Two ideas: 1. The guest is corrupting the vring or > submitting more requests than will fit into the ring. Somewhat unlikely > because it happens with both Windows and Linux guests. 2. QEMU's virtqueue > code is buggy, maybe the memory region cache which is used for fast guest RAM > accesses. Here is an expanded version of the debug patch which might help > identify which of these scenarios is likely. Sorry, it requires running the > guest again! This time let's make QEMU dump core so both QEMU state and guest > RAM are captured for further debugging. That way it will be possible to > extract more information using gdb without rerunning. Stefan --- diff --git > a/hw/virtio/virtio.c b/hw/virtio/virtio.c index a1ff647a66..28d89fcbcb 100644 > --- a/hw/virtio/virtio.c +++ b/hw/virtio/virtio.c @@ -866,6 +866,7 @@ void > *virtqueue_pop(VirtQueue *vq, size_t sz) return NULL; } rcu_read_lock(); + > uint16_t old_shadow_avail_idx = vq->shadow_avail_idx; if > (virtio_queue_empty_rcu(vq)) { goto done; } @@ -879,6 +880,12 @@ void > *virtqueue_pop(VirtQueue *vq, size_t sz) max = vq->vring.num; if (vq->inuse > >= vq->vring.num) { + fprintf(stderr, "vdev %p (\"%s\")\n", vdev, > vdev->name); + fprintf(stderr, "vq %p (idx %u)\n", vq, (unsigned int)(vq - > vdev->vq)); + fprintf(stderr, "inuse %u vring.num %u\n", vq->inuse, > vq->vring.num); + fprintf(stderr, "old_shadow_avail_idx %u last_avail_idx %u > avail_idx %u\n", old_shadow_avail_idx, vq->last_avail_idx, > vq->shadow_avail_idx); + fprintf(stderr, "avail %#" HWADDR_PRIx " avail_idx > (cache bypassed) %u\n", vq->vring.avail, virtio_lduw_phys(vdev, > vq->vring.avail + offsetof(VRingAvail, idx))); + fprintf(stderr, "used_idx > %u\n", vq->used_idx); + abort(); /* <--- core dump! */ virtio_error(vdev, > "Virtqueue size exceeded"); goto done; } > >
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