Re: [Qemu-devel] [PATCH 01/11] QMP: Introduce specification file

[Anthony Liguori <anthony@codemonkey.ws>]

Avi Kivity wrote:
>> An RPC is overkill unless you also introduce things like life cycle 
>> management.  Otherwise, you could never realistically interact with 
>> QEMU without a intermediary that handled life cycle management (e.g. 
>> libvirt).
>
> QMP is an RPC implemented over a text based protocol, with ad-hoc 
> marshalling.
>
> I agree that you need something to reap dead qemus, but I don't think 
> we can force libvirt on people.

QMP is an incremental improvement to the current monitor which people 
are consuming today.  It will have a pretty immediate and high rate of 
return.  libvirt can make use of it with probably very little change and 
it will immediately provide them with new features (async notifications) 
and better information about commands (error return values).

I have a really hard time not thinking this is the right thing to do 
because for relatively little churn we'll get a large return.  If you 
just exposed the monitor as an RPC, I don't think you really get 
anything more useful than QMP and you force a lot more change in libvirt 
which hurt the adoption of this protocol.

This is exactly what happened with Xend's transition from SExpr/HTTP 
(custom RPC) -> XML-RPC -> XenAPI.  libvirt still uses SExpr/HTTP 
because the cost of switching to the new way of doing things didn't 
warrant the return on investment.

s/libvirt/any other management tool that works today/g

Regards,

Anthony Liguori