Re: [Qemu-devel] [PATCH 0/3] client_migrate_switch and auto_switch (RHBZ 725009)

[Gerd Hoffmann <kraxel@redhat.com>]

On 08/26/11 12:03, Daniel P. Berrange wrote:
> On Fri, Aug 26, 2011 at 11:54:49AM +0200, Gerd Hoffmann wrote:
>> On 08/19/11 19:08, Alon Levy wrote:
>>> Fix the ticket expiration on target vm for a spice connection without introducing
>>> a race between the spice server switching the client to the new host itself and
>>> the target libvirt setting the new expiration date, by adding an option to
>>> client_migrate_info to not automatically switch the client on migration completion,
>>> instead waiting for an explicit client_migrate_switch (new monitor command) from
>>> libvirt.
>>
>> Hmm.  Guess the fundamental issue is that libvirt wants to use the
>> monitor to set the ticket instead of the command line for security
>> reasons.  The qemu monitor doesn't accept commands while the
>> incoming migration is running.  We also can't kick the incoming
>> migration via monitor, so first setting the ticket then start
>> migration doesn't work too.  Correct?
>
> There is actually a reliable window where we can use the monitor
> before incoming migration starts. Libvirt's migration is a 5 stage
> handshake:
>
>    1. Begin(src)
>        - Gets current source VM XML config
>    2. Prepare(dst)
>        - Launches QEMU -incoming
>        - Sets passwords, etc to monitor
>    3. Perform(src)

> Those stages are all serialized, so we can do anything we like
> with the QEMU monitor at stage 2, before stage 3 will start
> back on the src.

Ok, so I think we should be able to fix the race outlined above without 
adding new monitor commands, just by letting libvirt set the spice 
ticket in stage 2.

cheers,
   Gerd