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

[Daniel P. Berrange]

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)
      - Issue migrate_client_info
      - Issue migrate_set_speed
      - Issue migrate
      - Loop
          - Issue query-migrate
          - Break if finished/failed
  4. Finish(dst)
      - If success
          - start CPUs
        Else
          - Kill QEMU
  5. Confirm(src)
      - If success
          - Kill QEMU
        Else
          - Restart CPUs

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.

Regards,
Daniel
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