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Date: Sun, 22 Feb 2015 16:08:04 +0100
From: Alberto Garcia <berto@igalia.com>
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Subject: Re: [Qemu-devel] [PATCH 0/3] Support streaming to an intermediate
 layer
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To: Eric Blake <eblake@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>

On Fri, Feb 20, 2015 at 03:49:05PM -0700, Eric Blake wrote:

Hello and thanks for your comments.

> 1. implement QAPI introspection (we've been dreaming about this
> since qemu 1.5 days), then the caller just queries to see if the
> version of QMP has the optional 'top' parameter.

This sounds like the best option. Is there a roadmap? What's the
status?

I guess the amount of work depends a lot on the goals, but from what
I can see, something like a 'query-command-args' command that returns
the list of parameters is trivial, there's already the qmp_cmds array
in memory with all the necessary information.

> 2. implement the new feature as a new command (then the existing
> 'query-commands' becomes an easy probe; but we have code duplication
> with existing commands)

I don't think there would be a lot of code duplication because one
operation is a strict superset of the other, we would be just adding
unnecessary commands. I would go for option 1) if possible.

> 3. guarantee that we have sane error messages for bogus
> commands used as the probe. If {"execute":"block-stream",
> "data":{"device":"no-such","top":"bogus"}} gives a reliable
> DeviceNotFound error for qemu that supports optional 'top', and a
> reliable GenericError about an unknown argument 'top' in older qemu

It doesn't seem like the best solution considering the alternatives,
but for what it's worth:

Current QEMU:

(QEMU) block-stream device=no-such top=bogus
{u'error': {u'class': u'GenericError', u'desc': u"Invalid parameter 'top'"}}

With my patch:

(QEMU) block-stream device=no-such top=bogus
{u'error': {u'class': u'DeviceNotFound', u'desc': u"Device 'no-such' not found"}}

Regards,

Berto