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Re: [Qemu-devel] [PATCH for 2.11] virtio-net: don't touch virtqueue if v

From: Jason Wang
Subject: Re: [Qemu-devel] [PATCH for 2.11] virtio-net: don't touch virtqueue if vm is stopped
Date: Fri, 24 Nov 2017 10:57:11 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 2017年11月23日 18:59, Stefan Hajnoczi wrote:
On Thu, Nov 23, 2017 at 11:37:46AM +0800, Jason Wang wrote:
Guest state should not be touched if VM is stopped, unfortunately we
didn't check running state and tried to drain tx queue unconditionally
in virtio_net_set_status(). A crash was then noticed as a migration
destination when user type quit after virtqueue state is loaded but
before region cache is initialized. In this case,
virtio_net_drop_tx_queue_data() tries to access the uninitialized
region cache.

Fix this by only dropping tx queue data when vm is running.
hw/virtio/virtio.c:virtio_load() does the following:

   for (i = 0; i < num; i++) {
       if (vdev->vq[i].vring.desc) {
           uint16_t nheads;

            * VIRTIO-1 devices migrate desc, used, and avail ring addresses so
            * only the region cache needs to be set up.  Legacy devices need
            * to calculate used and avail ring addresses based on the desc
            * address.
           if (virtio_vdev_has_feature(vdev, VIRTIO_F_VERSION_1)) {
               virtio_init_region_cache(vdev, i);
           } else {
               virtio_queue_update_rings(vdev, i);

So the region caches should be initialized after virtqueue state is

It's unclear to me which code path triggers this issue.  Can you add a
backtrace or an explanation?


Migration coroutine was yield before region cache was initialized. The backtrace looks like:

#0  qio_channel_yield (ioc=0x55555758d000, condition=G_IO_IN) at io/channel.c:432 #1  0x0000555555b209c3 in channel_get_buffer (opaque=0x55555758d000, buf=0x555556f7c048 "", pos=1657701385, size=32768)
    at migration/qemu-file-channel.c:83
#2  0x0000555555b1f4a3 in qemu_fill_buffer (f=0x555556f7c010) at migration/qemu-file.c:293 #3  0x0000555555b1fd42 in qemu_peek_byte (f=0x555556f7c010, offset=0) at migration/qemu-file.c:553 #4  0x0000555555b1fd94 in qemu_get_byte (f=0x555556f7c010) at migration/qemu-file.c:566 #5  0x0000555555b1ffef in qemu_get_be32 (f=0x555556f7c010) at migration/qemu-file.c:646 #6  0x0000555555b1d002 in qemu_get_be32s (f=0x555556f7c010, pv=0x555557d6578c) at /home/devel/git/qemu/include/migration/qemu-file-types.h:78 #7  0x0000555555b1da0d in get_uint32 (f=0x555556f7c010, pv=0x555557d6578c, size=4, field=0x555556503ea0 <__compound_literal.0+416>)
    at migration/vmstate-types.c:241
#8  0x0000555555b1c0b5 in vmstate_load_state (f=0x555556f7c010, vmsd=0x555556375b60 <vmstate_virtio_pci_modern_queue_state>, opaque=0x555557d6577c,
    version_id=1) at migration/vmstate.c:140
#9  0x0000555555b1c090 in vmstate_load_state (f=0x555556f7c010, vmsd=0x555556375bc0 <vmstate_virtio_pci_modern_state_sub>, opaque=0x555557d604a0,
    version_id=1) at migration/vmstate.c:137
#10 0x0000555555b1cce5 in vmstate_subsection_load (f=0x555556f7c010, vmsd=0x555556375c20 <vmstate_virtio_pci>, opaque=0x555557d604a0)
    at migration/vmstate.c:453
#11 0x0000555555b1c199 in vmstate_load_state (f=0x555556f7c010, vmsd=0x555556375c20 <vmstate_virtio_pci>, opaque=0x555557d604a0, version_id=1)
    at migration/vmstate.c:160
#12 0x0000555555b00aa0 in virtio_pci_load_extra_state (d=0x555557d604a0, f=0x555556f7c010) at hw/virtio/virtio-pci.c:161 #13 0x00005555558651ab in get_extra_state (f=0x555556f7c010, pv=0x555557d68610, size=0, field=0x5555563cd0e0 <__compound_literal.4>)
    at /home/devel/git/qemu/hw/virtio/virtio.c:1808
#14 0x0000555555b1c0b5 in vmstate_load_state (f=0x555556f7c010, vmsd=0x5555562b7fe0 <vmstate_virtio_extra_state>, opaque=0x555557d68610, version_id=1)
    at migration/vmstate.c:140
#15 0x0000555555b1cce5 in vmstate_subsection_load (f=0x555556f7c010, vmsd=0x5555562b8160 <vmstate_virtio>, opaque=0x555557d68610)
    at migration/vmstate.c:453
#16 0x0000555555b1c199 in vmstate_load_state (f=0x555556f7c010, vmsd=0x5555562b8160 <vmstate_virtio>, opaque=0x555557d68610, version_id=1)
    at migration/vmstate.c:160
#17 0x0000555555865cc3 in virtio_load (vdev=0x555557d68610, f=0x555556f7c010, version_id=11) at /home/devel/git/qemu/hw/virtio/virtio.c:2110 #18 0x0000555555865705 in virtio_device_get (f=0x555556f7c010, opaque=0x555557d68610, size=0, field=0x5555563ca4a0 <__compound_literal.5>)
    at /home/devel/git/qemu/hw/virtio/virtio.c:1974
#19 0x0000555555b1c0b5 in vmstate_load_state (f=0x555556f7c010, vmsd=0x5555562b6ae0 <vmstate_virtio_net>, opaque=0x555557d68610, version_id=11)
    at migration/vmstate.c:140
#20 0x0000555555b15d20 in vmstate_load (f=0x555556f7c010, se=0x555557e81400) at migration/savevm.c:748 #21 0x0000555555b1827b in qemu_loadvm_section_start_full (f=0x555556f7c010, mis=0x555556564a40 <mis_current>) at migration/savevm.c:1903 #22 0x0000555555b185d5 in qemu_loadvm_state_main (f=0x555556f7c010, mis=0x555556564a40 <mis_current>) at migration/savevm.c:1998 #23 0x0000555555b187fc in qemu_loadvm_state (f=0x555556f7c010) at migration/savevm.c:2077 #24 0x0000555555b0c0af in process_incoming_migration_co (opaque=0x0) at migration/migration.c:327 #25 0x0000555555cc1d06 in coroutine_trampoline (i0=1469933216, i1=21845) at util/coroutine-ucontext.c:79
#26 0x00007ffff39d95d0 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#27 0x00007fffffffd2f0 in ?? ()
#28 0x0000000000000000 in ?? ()

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