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Re: [Qemu-devel] [RFC][PATCH v5 00/21] virtagent: host/guest RPC communi

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [RFC][PATCH v5 00/21] virtagent: host/guest RPC communication agent
Date: Fri, 10 Dec 2010 09:42:45 +0000

On Thu, Dec 9, 2010 at 9:03 PM, Anthony Liguori
<address@hidden> wrote:
> On 12/09/2010 02:45 PM, Michael Roth wrote:
>> On 12/08/2010 04:10 AM, Stefan Hajnoczi wrote:
>>> On Fri, Dec 3, 2010 at 6:03 PM, Michael Roth<address@hidden>
>>>  wrote:
>>>> These patches apply to master, and can also be obtained from:
>>>> git://repo.or.cz/qemu/mdroth.git virtagent_v5
>>> Why XML-RPC and not QMP?  When I skim through the patch series it
>>> seems like much of the work being done is very similar to QMP.
>> It does, actually, more than I realized. In terms of data encapsulation I
>> don't see why we couldn't use QMP/JSON for at least the current set of RPCs.
>> XMLRPC does support a wider range of data types however, such as nestable
>> arrays and structs, and set-precision numerical types like 32 and 64-bit
>> ints/floats, which may prove useful for future lower level interfaces.
> 1) XML-RPC is more widely supported than QMP (as there is zero support for
> QMP outside of QEMU and libvirt)

True, but the current design doesn't lend itself to custom agents at
the XML-RPC level.  Therefore using a standard protocol doesn't win
anything - it would have been more useful for QMP actually.

> 2) The target of this work is for guest agents

Not sure what this means.

> 3) QMP does not support bidirectional RPC messages.

Right.  QMP has commands, responses, and asynchronous events so the
client and server are two different roles (asymmetric).

I hadn't though through the details and this suggests shoehorning QMP
in as the agent protocol would be ugly.

> 4) The RPC mechanism is a minor part of virt-agent so ultimately, it kind of
> doesn't matter.  The RPC messages themselves are what's important.

Getting the design and features of virtagent right is key.  But we
still need to maintain the code, that's why I ask about reusing what
we've got.


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