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Re: [Xen-devel] Is fallback vhost_net to qemu for live migrate availableï

Hi Qin,

On Mon, Aug 26, 2013 at 10:32 PM, Qin Chuanyu <qinchuanyu@xxxxxxxxxx> wrote:
> Hi all
> I am participating in a project which try to port vhost_net on Xenã


> By change the memory copy and notify mechanism ïcurrently virtio-net with
> vhost_net could run on Xen with good performanceã

I think the key in doing this would be to implement a property
ioeventfd and irqfd interface in the driver domain kernel.  Just
hacking vhost_net with Xen specific knowledge would be pretty nasty

Did you modify the front end driver to do grant table mapping or is
this all being done by mapping the domain's memory?

> TCP receive throughput of
> single vnic from 2.77Gbps up to 6GpsãIn VM receive sideïI instead grant_copy
> with grant_map + memcopyïit efficiently reduce the cost of grant_table
> spin_lock of dom0ïSo the hole server TCP performance from 5.33Gps up to
> 9.5Gpsã
> Now I am consider the live migrate of vhost_net on Xenïvhost_net use
> vhost_log for live migrate on Kvmïbut qemu on Xen havn't manage the hole
> memory of VMïSo I am trying to fallback datapath from vhost_net to qemu when
> doing live migrate ïand fallback datapath from qemu to
> vhost_net again after vm migrate to new serverã

KVM and Xen represent memory in a very different way.  KVM can only
track when guest mode code dirties memory.  It relies on QEMU to track
when guest memory is dirtied by QEMU.  Since vhost is running outside
of QEMU, vhost also needs to tell QEMU when it has dirtied memory.

I don't think this is a problem with Xen though.  I believe (although
could be wrong) that Xen is able to track when either the domain or
dom0 dirties memory.

So I think you can simply ignore the dirty logging with vhost and it
should Just Work.

> My question isï
>         why didn't vhost_net do the same fallback operation for live migrate
> on KVMïbut use vhost_log to mark the dirty pageï
>         Is there any mechanism fault for the idea of fallback datapath from
> vhost_net to qemu for live migrateï

No, we don't have a mechanism to fallback  to QEMU for the datapath.
It would be possible but I think it's a bad idea to mix and match the


Anthony Liguori

> any question about the detail of vhost_net on Xen is welcomeã
> Thanks
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