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Re: [libvirt] [RFC] Vhost-user backends cross-version migration support

On 02/03/2017 04:34 PM, Michael S. Tsirkin wrote:
On Fri, Feb 03, 2017 at 03:11:10PM +0100, Maxime Coquelin wrote:

On 02/01/2017 09:35 AM, Maxime Coquelin wrote:

 Few months ago, Michael reported a problem about migrating VMs relying
on vhost-user between hosts supporting different backend versions:
 - Message-Id: <20161011173526-mutt-send-email-mst kernel org>
 - https://lists.gnu.org/archive/html/qemu-devel/2016-10/msg03026.html

 The goal of this thread is to draft a proposal based on the outcomes
of discussions with contributors of the different parties (DPDK/OVS

Thanks the first feedback. It seems to converge that this is Nova's
role, but not Libvirt one to manage these versions from management tool

I think the conclusion is not that it should go up the stack.  I think
this will just get broken all the time.  No one understands versions and
stuff. Even QEMU developers get confused and break compatibility once in
a while.

Is it that difficult for OVS to know with which versions of OVS it is
compatible with, and propose a way to be compatible with older versions?

From Nova perspective, it just has to collect supported versions
strings, and select the most recent common one.

My conclusion is that doing it from OVS side is wrong.  Migration is not
an OVS thing, it's a QEMU thing, and libvirt abstracts QEMU.    People
just want migration to work, ok? It's our job to do it, we do not really
need a "make things work" flag.

If libvirt does not want to use the vhost-user protocol (which sounds
reasonable, it's rather complex) how about qemu providing a small
utility to query the port?  We could output json or whatever.

Doing this, it would be libvirt role to compare key/value pairs in the json output, right? If so, it would mean teaching libvirt to compare
things it knows nothing about, that are likely to evolve over time.

Also, it means the port will have already been created, so I'm not
clear how we would manage compatibility version.

This can help with MTU as well.

And maybe it will help with nowait support - if someone uses the utility
to dump backend config once, QEMU can later start the device without
feature queries.


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