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



On 12/15/2011 07:57 AM, Luiz Capitulino wrote:
On Thu, 15 Dec 2011 13:02:40 +0000
Stefan Hajnoczi<stefanha 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.

Regards,

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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