Re: [PATCH v3 1/6] migration: Allow immutable device state to be migrated early (i.e., before RAM)

[David Hildenbrand <david@redhat.com>]

On 05.01.23 02:27, Michael S. Tsirkin wrote:
> On Fri, Dec 23, 2022 at 10:34:36AM +0100, David Hildenbrand wrote:
>> On 22.12.22 12:02, David Hildenbrand wrote:
>>> For virtio-mem, we want to have the plugged/unplugged state of memory
>>> blocks available before migrating any actual RAM content. This
>>> information is immutable on the migration source while migration is active,
>>>
>>> For example, we want to use this information for proper preallocation
>>> support with migration: currently, we don't preallocate memory on the
>>> migration target, and especially with hugetlb, we can easily run out of
>>> hugetlb pages during RAM migration and will crash (SIGBUS) instead of
>>> catching this gracefully via preallocation.
>>>
>>> Migrating device state before we start iterating is currently impossible.
>>> Introduce and use qemu_savevm_state_start_precopy(), and use
>>> a new special migration priority -- MIG_PRI_POST_SETUP -- to decide whether
>>> state will be saved in qemu_savevm_state_start_precopy() or in
>>> qemu_savevm_state_complete_precopy_*().
>>>
>>> We have to take care of properly including the early device state in the
>>> vmdesc. Relying on migrate_get_current() to temporarily store the vmdesc is
>>> a bit sub-optimal, but we use that explicitly or implicitly all over the
>>> place already, so this barely matters in practice.
>>>
>>> Note that only very selected devices (i.e., ones seriously messing with
>>> RAM setup) are supposed to make use of that.
>>>
>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>
>> [...]
>>
>>>        if (inactivate_disks) {
>>> @@ -1427,6 +1474,10 @@ int qemu_savevm_state_complete_precopy_non_iterable(QEMUFile *f,
>>>            qemu_put_buffer(f, (uint8_t *)json_writer_get(vmdesc), vmdesc_len);
>>>        }
>>> +    /* Free it now to detect any inconsistencies. */
>>> +    g_free(vmdesc);
>>
>> Missed to convert that to a json_writer_free().
> 
> 
> I get it you will post v4?

Yes, once the discussions on this version are done.

-- 
Thanks,

David / dhildenb