Re: [Qemu-devel] [PATCH v3 8/9] qmp: add query-block-jobs

[Kevin Wolf]

Am 15.12.2011 13:00, schrieb Stefan Hajnoczi:
> On Thu, Dec 15, 2011 at 11:30 AM, Luiz Capitulino
> <address@hidden> wrote:
>> On Thu, 15 Dec 2011 11:34:07 +0100
>> Kevin Wolf <address@hidden> wrote:
>>
>>> Am 15.12.2011 09:27, schrieb Stefan Hajnoczi:
>>>> On Wed, Dec 14, 2011 at 03:54:52PM +0100, Kevin Wolf wrote:
>>>>> Am 13.12.2011 14:52, schrieb Stefan Hajnoczi:
>>>>>> diff --git a/hmp.c b/hmp.c
>>>>>> index 66d9d0f..c16d6a1 100644
>>>>>> --- a/hmp.c
>>>>>> +++ b/hmp.c
>>>>>> @@ -499,6 +499,46 @@ void hmp_info_pci(Monitor *mon)
>>>>>>      qapi_free_PciInfoList(info);
>>>>>>  }
>>>>>>
>>>>>> +void hmp_info_block_jobs(Monitor *mon)
>>>>>> +{
>>>>>> +    BlockJobInfoList *list;
>>>>>> +    Error *err = NULL;
>>>>>> +
>>>>>> +    list = qmp_query_block_jobs(&err);
>>>>>> +    assert(!err);
>>>>>> +
>>>>>> +    if (!list) {
>>>>>> +        monitor_printf(mon, "No active jobs\n");
>>>>>> +        return;
>>>>>> +    }
>>>>>> +
>>>>>> +    while (list) {
>>>>>> +        /* The HMP output for streaming jobs is special because 
>>>>>> historically it
>>>>>> +         * was different from other job types so applications may 
>>>>>> depend on the
>>>>>> +         * exact string.
>>>>>> +         */
>>>>>
>>>>> Er, what? This is new code. What HMP clients use this string? I know
>>>>> that libvirt already got support for this before we implemented it, but
>>>>> shouldn't that be QMP only?
>>>>
>>>> Libvirt HMP uses this particular string, which turned out to be
>>>> sub-optimal once I realized we might support other types of block jobs
>>>> in the future.
>>>>
>>>> You can still build libvirt HMP-only by disabling the yajl library
>>>> dependency.  The approach I've taken is to make the interfaces available
>>>> over both HMP and QMP (and so has the libvirt-side code).
>>>>
>>>> In any case, we have defined both HMP and QMP.  Libvirt implements both
>>>> and I don't think there's a reason to provide only QMP.
>>>>
>>>> Luiz: For future features, are we supposed to provide only QMP
>>>> interfaces, not HMP?
>>>
>>> Of course, qemu should provide them as HMP command. But libvirt
>>> shouldn't use HMP commands. HMP is intended for human users, not as an
>>> API for management.
>>
>> That's correct.
>>
>> What defines if you're going to have a HMP version of a command is if
>> you expect it to be used by humans and if you do all its output and
>> arguments should be user friendly. You should never expect nor assume
>> it's going to be used by a management interface.
> 
> Okay, thanks Kevin and Luiz for explaining.  In this case I know it is
> used by a management interface because libvirt has code to use it.
> 
> I was my mistake to include the HMP side as part of the "API" but it's
> here now and I think we can live with this.

We probably can, but I would prefer fixing it in libvirt. Possibly the
right fix there would be to remove it entirely from the HMP code - if I
understand right, the HMP code is only meant to support older qemu
versions anyway.

I also don't quite understand why there even is an option to disable QMP
in libvirt, we have always told that HMP will become unstable.

Kevin