Re: [PATCH v2 3/3] migration: Postpone postcopy preempt channel to be after main

[Juan Quintela]

Peter Xu <peterx@redhat.com> wrote:
> Postcopy with preempt-mode enabled needs two channels to communicate.  The
> order of channel establishment is not guaranteed.  It can happen that the
> dest QEMU got the preempt channel connection request before the main
> channel is established, then the migration may make no progress even during
> precopy due to the wrong order.
>
> To fix it, create the preempt channel only if we know the main channel is
> established.
>
> For a general postcopy migration, we delay it until postcopy_start(),
> that's where we already went through some part of precopy on the main
> channel.  To make sure dest QEMU has already established the channel, we
> wait until we got the first PONG received.  That's something we do at the
> start of precopy when postcopy enabled so it's guaranteed to happen sooner
> or later.
>
> For a postcopy recovery, we delay it to qemu_savevm_state_resume_prepare()
> where we'll have round trips of data on bitmap synchronizations, which
> means the main channel must have been established.
>
> Signed-off-by: Peter Xu <peterx@redhat.com>

Reviewed-by: Juan Quintela <quintela@redhat.com>

But I still think that our channel setup is overcomplex (famous last
words, specially thinking that I designed it, sniff).

Later, Juan.