Re: [RFC PATCH] tests/qtest/migration: Add cpu hotplug test

[Fabiano Rosas <farosas@suse.de>]

Stefan Hajnoczi <stefanha@gmail.com> writes:

> On Mon, 13 Jan 2025 at 16:09, Fabiano Rosas <farosas@suse.de> wrote:
>>
>> Bug #2594 is about a failure during migration after a cpu hotplug. Add
>> a test that covers that scenario. Start the source with -smp 2 and
>> destination with -smp 3, plug one extra cpu to match and migrate.
>>
>> The issue seems to be a mismatch in the number of virtqueues between
>> the source and destination due to the hotplug not changing the
>> num_queues:
>>
>>   get_pci_config_device: Bad config data: i=0x9a read: 4 device: 5
>>   cmask: ff wmask: 0 w1cmask:0
>>
>> Usage:
>> $ QTEST_QEMU_IMG=./qemu-img QTEST_QEMU_BINARY=./qemu-system-x86_64 \
>>   ./tests/qtest/migration-test -p /x86_64/migration/hotplug/cpu
>>
>> References: https://gitlab.com/qemu-project/qemu/-/issues/2594
>> References: https://issues.redhat.com/browse/RHEL-68302
>> Signed-off-by: Fabiano Rosas <farosas@suse.de>
>> ---
>> As you can see there's no fix attached to this. I haven't reached that
>> part yet, suggestions welcome =). Posting the test case if anyone
>> wants to play with this.
>>
>> (if someone at RH is already working on this, that's fine. I'm just
>> trying to get some upstream bugs to move)
>
> The management tool should set num_queues on the destination to ensure
> migration compatibility.
>

I'm not sure that's feasible. The default num-queues seem like an
implementation detail that the management application would not have a
way to query. Unless it starts the source with a fixed number that
already accounts for all hotplug/unplug operations during the VM
lifetime, which would be wasteful in terms of resources allocated
upfront.

That would also make the destination run with a suboptimal (< #vcpus)
number of queues, although that's already the case in the source after
the hotplug. Do we have any definition on what should happen durgin
hotplug? If one plugs 100 vcpus, should num-queues remain as 2?