[Qemu-devel] How live migration work for vhost-user

[fengyd]

Hi,

During live migration,  the folloing log can see in nova-compute.log in my
environment:
 ERROR nova.virt.libvirt.driver [req-039a85e1-e7a1-4a63-bc6d-c4b9a044aab6
0cdab20dc79f4bc6ae5790e7b4a898ac 3363c319773549178acc67f32c78310e - default
default] [instance: 5ec719f4-1865-4afe-a207-3d9fae22c410] Live Migration
failure: internal error: qemu unexpectedly closed the monitor:
2019-04-15T02:58:22.213897Z qemu-kvm: VQ 0
size 0x100 < last_avail_idx 0x1e - used_idx 0x23

It's OK for standard Linux VM, but not OK for our VM where virtio is
implemented by ourself.
KVM version as follow:
qemu-kvm-common-ev-2.12.0-18.el7_6.3.1.x86_64
qemu-kvm-ev-2.12.0-18.el7_6.3.1.x86_64
libvirt-daemon-kvm-3.9.0-14.2.el7.centos.ncir.8.x86_64

Do you know what's the difference between virtio and vhost-user during
migration?
The function virtio_load in Qemu is called for virtio and vhost-user during
migration.
For virtio,  last_avail_idx  and used_idx are stored in Qemu, Qemu is
responsible for updating their values accordingly
For vhost-user, last_avail_idx  and used_idx are stored in vhost-user app,
eg. DPDK, not in Qemu?
How does migration work for vhost-user?

Thanks in advance
Yafeng