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Re: [libvirt] [Qemu-devel] QMP: Supporting off tree APIs



On 01/10/2012 02:55 PM, Luiz Capitulino wrote:
On Tue, 10 Jan 2012 13:18:41 -0600
Anthony Liguori<anthony codemonkey ws>  wrote:

On 01/06/2012 01:42 PM, Luiz Capitulino wrote:
On Fri, 06 Jan 2012 09:08:19 -0600
We also need to look at this interface as a public interface whether we
technically committed it to or not.  The fact is, an important user is relying
upon so that makes it a supported interface.  Even though I absolutely hate it,
this is why we haven't changed the help output even after all of these years.
Not breaking users should be one of our highest priorities.

One thing I don't understand: how is libvirt relying on it if it doesn't
exist in qemu.git yet?

Because there was a discussion on qemu-devel and we agreed on an interface that
both sides would implement to.

I expect this to happen more often in the future too.

For future commands we either, implement it right away or ask libvirt to
wait for the command to be merged, or at least pass initial review.

Or commit the schema entry to qapi-schema.json with gen=False.

But when this command was first proposed, qapi-schema.json didn't exist in the tree :-)

But aren't we going to introduce a proper async interface?  This is what has me
confused.

Yes, I was thinking about new block commands using this API before we get
proper async support...

Well let's avoid that problem by doing proper async support before we get new block commands ;-)

There's more, because I skipped this review in v3 as I jumped to the
"proper async support" discussion...

Well let's do proper async support.  As I mentioned, I'd rather take this
command in as-is, introduce proper async support, and then deprecate a bunch of
stuff at once.

Ok, I'm willing to do this as Stefan has agreed on deprecating this as soon as
we get proper async support.

Excellent.

BTW, you mentioned that you were going to send an RFC for proper async support?

Regards,

Anthony Liguori


What I'd suggest is that we take the command in as-is and we mark it:

Since: 1.1
Deprecated: 1.2
See Also: TBD

The idea being that we'll introduce new generic async commands in 1.2 and
deprecate this command.  We can figure out the removal schedule then too.  Since
this command hasn't been around all that long, we can probably have a short
removal schedule.

That makes its inclusion even discussable :) A few (very honest) questions:

   1. Is it really worth it to have the command for one or two releases?

Yes.  The most important consideration IMHO is parallelizing development.  We
want the block layer to evolve to the greatest extent possible independent of
our other subsystems.  If we don't have the right infrastructure in QMP to
support a block feature, that shouldn't hold up progress in the block layer to
the greatest extent possible.

   2. Will we allow other block commands to use this async API?

Depends on how long it takes to do "proper async support".

   3. Are we going to accept other ad-hoc async APIs until we have a
      proper one?

Yes.  So let's get serious about getting a proper interface in :-)

ACK


Regards,

Anthony Liguori




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