[libvirt] [Qemu-devel] Transitioning from HMP to QMP for QEMU

Anthony Liguori anthony at codemonkey.ws
Thu Dec 15 14:06:41 UTC 2011

On 12/15/2011 07:57 AM, Luiz Capitulino wrote:
> On Thu, 15 Dec 2011 13:02:40 +0000
> Stefan Hajnoczi<stefanha at gmail.com>  wrote:
>> What is the status of QEMU's transition from HMP to the QMP interface?
> Depends on what you consider the transition to be.
> For management tools the transition can be considered done already because we
> do not support HMP as a stable interface.
>> My current understanding is that QEMU provides new HMP commands for
>> humans, but HMP is being phased out as an API.
> It already did.
>>   Management tools
>> should rely only on QMP for new commands.  That would mean new HMP
>> commands are not guaranteed to produce backwards-compatible output
>> because tools are not supposed to parse the output.
> Exactly.
>> On the libvirt side, new QEMU features should only be supported via
>> the json monitor in the future (i.e. human monitor patches should not
>> be sent/merged)? Existing HMP commands will still need the human
>> monitor support in order to handle old QEMU versions gracefully, but
>> I'm thinking about new commands only.
> Maybe it's a matter of terminology, but I have the impression you're
> talking about two things here:
>   1. HMP will always exist, in the meaning that qemu will always provide
>      a human interface. If we move it to a python script or some kind of
>      external process, that's an implementation detail.
>      This means that, if you're adding new functionality to qemu and it
>      does make sense for humans to use it, then it should have a HMP
>      version.
>   2. If you do add the HMP interface, that's for humans to consume and
>      its output/semantics should make sense for humans, not for management tools.

3. All HMP commands will be implemented in terms of QMP commands (and only in 
terms of QMP commands).

There are still a lot of HMP commands that don't have an QMP analog.  Luiz, it 
might make sense to setup a wiki page which instructions on how to convert an 
HMP command to a QMP command using QAPI.

If we did that, we could probably get more folks involved in the conversion process.


Anthony Liguori

>> Does everyone agree on this?  I think this is an important discussion
>> if we want our management interface to get better and more consistent
>> in the future.
>> Stefan

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