Re: [PATCH v8 4/7] qapi: add x-blockdev-replace command

[Markus Armbruster]

Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru> writes:

> On 18.10.23 13:45, Markus Armbruster wrote:
>> Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru> writes:
>> 
>>> Add a command that can replace bs in following BdrvChild structures:
>>>
>>>   - qdev blk root child
>>>   - block-export blk root child
>>>   - any child of BlockDriverState selected by child-name
>>>
>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
>
> [..]
>
>>> --- /dev/null
>>> +++ b/stubs/blk-by-qdev-id.c
>>> @@ -0,0 +1,9 @@
>>> +#include "qemu/osdep.h"
>>> +#include "qapi/error.h"
>>> +#include "sysemu/block-backend.h"
>>> +
>>> +BlockBackend *blk_by_qdev_id(const char *id, Error **errp)
>>> +{
>>> +    error_setg(errp, "blk '%s' not found", id);
>>
>> Is this expected to happen?
>
> Yes, if call the command from qemu-storage-daemon, where qdev-monitor is not 
> linked in.

It happens when you try to x-blockdev-replace with "parent-type":
"qdev".  Correct?

> Maybe, better message would be
>
>    "devices are not supported"

Best to spell out which argument is the problem.

Stupidest solution that could possibly work:

    "Parameter 'parent-type' does not accept value 'qdev'"

This is exactly what we'd get if we compiled out the parts that don't
make sense for qemu-storage-daemon.

> Maybe, that possible to use some 'if': notation in qapi, to not include 
> support for qdev into the new command, when it compiled into 
> qemu-storage-daemon? Seems that would not be simple, as we also need to split 
> compilation of the command somehow, now it compiled once both for qemu and 
> qemu tools..

That's precisely the problem.

Our reuse of parts of qemu-system-FOO's QAPI schema for
qemu-storage-daemon isn't pretty, but has worked for us so far.

>>> +    return NULL;
>>> +}
>> [...]
>> QAPI schema
>> Acked-by: Markus Armbruster <armbru@redhat.com>
>>