Re: [PATCH 2/6] migration: Kick postcopy threads on cancel

[Peter Xu <peterx@redhat.com>]

On Mon, Dec 02, 2024 at 07:01:33PM -0300, Fabiano Rosas wrote:
> Make sure postcopy threads are released when migrate_cancel is
> issued. Kick the postcopy_pause semaphore and have the fault thread
> read 'fault_thread_quit' when joining.
> 
> While here fix the comment mentioning userfault_event_fd.
> 
> Signed-off-by: Fabiano Rosas <farosas@suse.de>

I remember when working on postcopy, I thought about failing migrate-cancel
for postcopy in general, rejecting such request.  And when working on the
recover feature, there's no concern on having it being cancelled, because
the user really shouldn't do that..

The problem is migrate-cancel means crashing the VM on both sides when QEMU
already goes into postcopy stage.

If the user wants to crash the VM anyway, an easier way to do is killing on
both sides.

If the user wished to cancel, we should tell them "postcopy cannot be
cancelled, until complete".  That's probably the major reason why people
think postcopy is dangerous to use..

Or do we have any use case this could be a valid scenario?

-- 
Peter Xu