Re: [Qemu-devel] unplug_request and migration

[Juan Quintela <quintela@redhat.com>]

David Gibson <david@gibson.dropbear.id.au> wrote:
> On Fri, Jun 09, 2017 at 11:09:10AM +0200, Igor Mammedov wrote:
>> On Fri, 9 Jun 2017 00:41:06 +1000
>> David Gibson <david@gibson.dropbear.id.au> wrote:
>> 
>> > Hi Dave & Juan,
>> > 
>> > I'm hoping one of you can answer this.
>> > 
>> > I'm currently grappling with (amongst other things) a pseries machine
>> > racing a hot unplug operation with a migrate.  There's various issues
>> > with what interim state we need, and which bits of it need to be
>> > migrated that I'm still investigating.  But, there's a more general
>> > question that I'm guessing must have already been addressed for x86.
>> > 
>> > For any "soft" unplug device - i.e. using ->unplug_request, rather
>> > than ->unplug, giving a device_del command will just ask the guest
>> > nicely to release the device, with the completion of the unplug
>> > happening only if and when the guest indicates it's ready for the
>> > device to go away.  AFAICT, the device_del command will return as soon
>> > as the request is made, but if the guest is busy, the completion of
>> > the hot unplug could take arbitrarily long.
>> > 
>> > So, what happens if there's a migration in between the unplug_request
>> > and the guest completing the unplug?  How does libvirt (or whatever)
>> > know whether to include the device on the destination machine command
>> > line?
>> > 
>> 
>> looking at qdev_unplug():
>>     if (!migration_is_idle()) { 
>>         error_setg(errp, "device_del not allowed while migrating");
>>         return;
>>     }
>> 
>> so unplug request should fail if migration is in progress , it won't reach guest
>> and mgmt side will have to repeat request on migration completion.
>> 
>> But it's still possible to issue unplug request first and then start
>> migration,
>
> Right, that's the case I'm interested in, not the other way around.
>
>> that's where race between DEVICE_DELETED and migration start (starting DST with
>> being unplugged device) occurs.
>> 
>> it could be possible:
>>  1: on unplug_request() set global flag that there is pending unplug and forbid
>>     migration until completion. But there is no guarantee that unplug will
>>     be completed nor a way to notice that it's failed/rejected by guest.
>>     I'm not sure how that could be solved.
>>  2: set per device pending_unplug flag and delay unplug event from guest
>>     until migration is completed if migration is in progress when unplug
>>     callback is called.
>>     mgmt will treat the case as usual migration, i.e. start dst with being
>>     unplugged device, and device will be removed on dst side on migration
>>     completion.
>>     (it should be generic solution as x86 is also affected), as place where
>>     to put this common logic I'd suggest hotplug_handler_unplug()
>
> So.. it seems like the short version is that racing migration and
> unplug is broken already.

> Which is unfortunate, but at least means I don't need to worry about
> it particularly for Power.

Yeap.  I think that when I put the patches (for 2.10) to disable
hot[un]plug during migration, it was the 1st try to do something about
it.

Later, Juan.