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Re: [Qemu-devel] Proposal for extensions of block job commands in QEMU 1

From: Anthony Liguori
Subject: Re: [Qemu-devel] Proposal for extensions of block job commands in QEMU 1.2
Date: Mon, 21 May 2012 10:44:20 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:11.0) Gecko/20120329 Thunderbird/11.0.1

On 05/21/2012 09:47 AM, Paolo Bonzini wrote:
Il 21/05/2012 16:40, Anthony Liguori ha scritto:
On 05/21/2012 09:26 AM, Paolo Bonzini wrote:
Il 21/05/2012 16:19, Anthony Liguori ha scritto:

I'm not against it in principle, just in practice.  Today, checking
whether a command exists is:

commands = qmp.query_commands()

if 'block-stream' in commands:
      # has block-stream

I have a hard time envisioning how schema introspection can be
reasonably implemented in a client.

schema = qmp.query_command_schema('block-stream')

What would schema return?

Did you mean:

if schema['arguments'].has_key('on_error'):

Yes, something like that.

What about adding a parameter to a structure?

schema = qmp.query_type('foo')
if schema['data'].has_key('xyz')

This may end up being workable. I hadn't thought of having granular QMP schema querying functions.

BTW, the other problem with adding arguments like this is that it makes
a stable C API impossible.

Adding fields to structs is not a problem as long as "libqmp" takes care
of all allocations on part of its client.

Yes, the previous version I wrote handled this padding structures appropriately.

Adding parameters to commands
requires some smartness, but there are ways to do it:

1) add the first version number to the schema, generate versioned entry


etc.  Provide multiple headers libqmp-1.1.h, libqmp-1.2.h etc. that take
care of #define'ing qmp_block_stream to one of them

This is pretty nasty :-/ It also gets very challenging if some options are backported and others aren't.

I guess let me ask the following question:

Is the problem that should have an 'options' parameter to this command? Do we anticipate future addition of arguments? If we can at least minimize the C API breakages, I'd be happy.


Anthony Liguori

2) Same as (1) but use qmp_block_stream for the oldest version having
the command.

3) have fun with the preprocessor (macro with variable arguments,
sizeof, designated initializers, whatever) and emulate keyword arguments
for the C API.


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