Re: [PATCH RFC] memory: Don't allow to resize RAM while migrating

[David Hildenbrand]

On 14.02.20 11:42, Dr. David Alan Gilbert wrote:
> * David Hildenbrand (address@hidden) wrote:
>> On 14.02.20 11:25, Dr. David Alan Gilbert wrote:
>>> * David Hildenbrand (address@hidden) wrote:
>>>> Resizing while migrating is dangerous and does not work as expected.
>>>> The whole migration code works on the usable_length of ram blocks and does
>>>> not expect this to change at random points in time.
>>>>
>>>> Precopy: The ram block size must not change on the source, after
>>>> ram_save_setup(), so as long as the guest is still running on the source.
>>>>
>>>> Postcopy: The ram block size must not change on the target, after
>>>> synchronizing the RAM block list (ram_load_precopy()).
>>>>
>>>> AFAIKS, resizing can be trigger *after* (but not during) a reset in
>>>> ACPI code by the guest
>>>> - hw/arm/virt-acpi-build.c:acpi_ram_update()
>>>> - hw/i386/acpi-build.c:acpi_ram_update()
>>>>
>>>> I see no easy way to work around this. Fail hard instead of failing
>>>> somewhere in migration code due to strange other reasons. AFAIKs, the
>>>> rebuilts will be triggered during reboot, so this should not affect
>>>> running guests, but only guests that reboot at a very bad time and
>>>> actually require size changes.
>>>>
>>>> Let's further limit the impact by checking if an actual resize of the
>>>> RAM (in number of pages) is required.
>>>>
>>>> Don't perform the checks in qemu_ram_resize(), as that's called during
>>>> migration when syncing the used_length. Update documentation.
>>>
>>> Interesting; we need to do something about this - but banning resets
>>> during migration is a bit harsh; and aborting the source VM is really
>>> nasty - for a precopy especially we shouldn't kill the source VM,
>>> we should just abort the migration.
>>
>> Any alternative, easy solutions to handle this? I do wonder how often
>> this will actually trigger in real life.
> 
> Well it's not that hard to abort a migration (I'm not sure we've got a
> convenient wrapper to do it - but it shouldn't be hard to add).
> 

We do have qmp_migrate_cancel(). I hope that can be called under BQL.

Can that be called in both, precopy and postcopy case? I assume in the
precopy, it's easy.

-- 
Thanks,

David / dhildenb