Re: [PATCH RFC 5/9] migration: Thread-ify precopy vmstate load process

[Peter Xu <peterx@redhat.com>]

On Thu, Sep 04, 2025 at 01:38:14AM +0000, Dr. David Alan Gilbert wrote:
> > > > In general, QEMUFile IOs should not need BQL, that is when receiving the
> > > > VMSD data and waiting for e.g. the socket buffer to get refilled.  But
> > > > that's the easy part.
> > > 
> > > It's probably generally a good thing to get rid of the BQL there, but I bet
> > > it's going to throw some surprises; maybe something like devices doing
> > > stuff before the migration has fully arrived
> > 
> > Is that pre_load() or.. maybe something else?
> > 
> > I should still look into each of them, but only if we want to further push
> > the bql to be at post_load() level.  I am not sure if some pre_load() would
> > assume BQL won't be released until post_load(), if so that'll be an issue,
> > and that will need some closer code observation...
> 
> Well maybe pre_load; but anything that might start happening once the
> state has been loaded that shouldn't start happening until migration ends;
> I think there are some devices that do it properly and wait for end of migration.
> 
> > > or incoming socket connections to non-migration stuff perhaps.
> > 
> > Any example for this one?
> 
> I was just thinking aloud; but was thinking of NIC activity or maybe
> UI stuff?  But just guesses.

I'll see if I can get some more test coverage to be slightly more
confident..  If you have any special setup / devices in mind that might be
prone to issues with this series, please let me know!  I can test them even
earlier.

We also always have the option to provide a knob, so that whenever
necessary we can still allow user to fallback to the coroutine way, until
it's proven solid..

Thanks,

-- 
Peter Xu