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

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] Transitioning from HMP to QMP for QEMU
Date: Thu, 15 Dec 2011 13:45:57 +0000

On Thu, Dec 15, 2011 at 1:39 PM, Jan Kiszka <address@hidden> wrote:
> On 2011-12-15 14:38, Lucas Meneghel Rodrigues wrote:
>> On 12/15/2011 11:33 AM, Kevin Wolf wrote:
>>> Am 15.12.2011 14:18, schrieb Jan Kiszka:
>>>> On 2011-12-15 14:02, Stefan Hajnoczi wrote:
>>>>> What is the status of QEMU's transition from HMP to the QMP interface?
>>>>> My current understanding is that QEMU provides new HMP commands for
>>>>> humans, but HMP is being phased out as an API.  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.
>>>>> 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.
>>>>> 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.
>>>> To phase out the classic HMP implementation, we need an internal
>>>> HMP-over-JSON wrapper (with tab expansion etc.) so that virtual console
>>>> and gdbstub monitors continue to benefit from new commands. Those
>>>> interfaces will stay for a long time, I'm sure.
>>> I think we're not talking about dropping HMP here, only about how long
>>> to support it as a stable API for management tools. I believe that we
>>> have been in a transitional phase for long enough now that we can start
>>> changing the output format of HMP commands without considering it an API
>>> breakage.
>> Yes, I've got the same impression. But while we are at it, forgive my
>> naiveness, but wouldn't be worthwhile to consider dropping the human
>> monitor in the long run?
> Surely not the interface (for virtual console & gdbstub), but the
> internal implementation I hope.

I wonder if we could shape up qemu/QMP/qmp-shell.py to the level where
it provides HMP human-friendliness and then drop HMP.

This is a side-discussion though.  The main point was to assert that
HMP is not a management interface and that we don't add text monitor
support new commands to libvirt.  It doesn't solve the problem of
having to implement each command twice today in libvirt but it at
least establishes the flexibility to morph HMP into a layer on top of
QMP in the future in QEMU.


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