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

[Dr. David Alan Gilbert]

* fengyd (address@hidden) wrote:
> Hi,
> 
> For vhost,  *last_avail_idx*  is maintained in vhost_virtqueue
> but during live migration, *last_avail_idx*  is fetched from VirtQueue.
> Do you know how these two *last_avail_idx *are synchronized?
> 
> virtio_load related code which is called during live migration:
> 
> * vdev->vq[i].inuse = (uint16_t)(vdev->vq[i].last_avail_idx -*
> *                                vdev->vq[i].used_idx);*
> *            if (vdev->vq[i].inuse > vdev->vq[i].vring.num) {*
> *                error_report("VQ %d size 0x%x < last_avail_idx 0x%x - "*
> *                             "used_idx 0x%x",*
> *                             i, vdev->vq[i].vring.num,*
> *                             vdev->vq[i].last_avail_idx,*

I don't know that code well; but I think the answer is that since the
queues themselves are in guest memory, the guest memory is migrated
by the normal migration code and so the queues version of last_avail_idx
should be correct.  The 'log' mechanism I previously mentioned will
need to make sure the queue pages are marked dirty to make sure these
are updated correctly.

Dave

> 
> 
> Thanks
> 
> On Tue, 23 Apr 2019 at 14:20, fengyd <address@hidden> wrote:
> 
> > Hi,
> >
> > I want to add some log to qemu-kvm-ev.
> > Do you know how to compile qemu-kvm-ev from source code?
> >
> > Thanks
> >
> > Yafeng
> >
> > On Tue, 16 Apr 2019 at 16:47, Dr. David Alan Gilbert <address@hidden>
> > wrote:
> >
> >> * fengyd (address@hidden) wrote:
> >> > ---------- Forwarded message ---------
> >> > From: fengyd <address@hidden>
> >> > Date: Tue, 16 Apr 2019 at 09:17
> >> > Subject: Re: [Qemu-devel] How live migration work for vhost-user
> >> > To: Dr. David Alan Gilbert <address@hidden>
> >> >
> >> >
> >> > Hi,
> >> >
> >> > Any special feature needs to be supported on guest driver?
> >> > Because it's OK for standard Linux VM, but not OK for our VM where
> >> virtio
> >> > is  implemented by ourself.
> >>
> >> I'm not sure; you do have to support that 'log' mechanism but I don't
> >> know what else is needed.
> >>
> >> > And with qemu-kvm-ev-2.6, live migration can work with our VM where
> >> virtio
> >> > is  implemented by ourself.
> >>
> >> 2.6 is pretty old, so there's a lot of changes - not sure what's
> >> relevant.
> >>
> >> Dave
> >>
> >> > Thanks
> >> > Yafeng
> >> >
> >> > On Mon, 15 Apr 2019 at 22:54, Dr. David Alan Gilbert <
> >> address@hidden>
> >> > wrote:
> >> >
> >> > > * fengyd (address@hidden) 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 / address@hidden / Manchester, UK
> >> > >
> >> --
> >> Dr. David Alan Gilbert / address@hidden / Manchester, UK
> >>
> >
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK