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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* fengyd (fengyd81@gmail.com) wrote:
> 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?

I don't know the details, but my understanding is that vhost-user
tells the vhost-user client about an area of 'log' memory, where the
vhost-user client must mark pages as dirty.

In the qemu source, see docs/interop/vhost-user.txt and see
the VHOST_SET_LOG_BASE and VHOST_USER_SET_LOG_FD calls.

If the client correctly marks the areas as dirty, then qemu
should resend those pages across.


Dave

> Thanks in advance
> Yafeng
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK