Re: [Qemu-devel] [PATCH 2/3] QMP: Introduce Human Monitor passthrough command

[Daniel P. Berrange]

On Thu, Nov 11, 2010 at 10:30:47AM -0600, Anthony Liguori wrote:
> On 11/11/2010 09:55 AM, Daniel P. Berrange wrote:
> >On Thu, Nov 11, 2010 at 04:47:41PM +0100, Markus Armbruster wrote:
> >>>>>  3. Query Commands
> >>>>>  =================
> >>>>>           
> >>>>In the real human monitor, cpu-index is state (Monitor member mon_cpu).
> >>>>For pass through, you shift that state into the client (argument
> >>>>cpu-index).  Is there any other state that could need shifting?  You
> >>>>mention getfd.
> >>>>         
> >>>Surprisingly or not, this is a very important question for QMP itself.
> >>>
> >>>Anthony has said that we should make it stateless, and I do think this
> >>>is good because it seems to simplify things considerably.
> >>>
> >>>However, I haven't thought about how to make things like getfd stateless.
> >>>       
> >>Hmm, that sounds like we should investigate the getfd problem sooner
> >>rather than later.
> >>     
> >The SCM_RIGHTS code allows you to send/receive multiple file handles in a
> >single sendmsg/recvmsg  call. So why don't we just allow sending of the
> >file handles with the monitor command that actually needs them, instead of
> >ahead of time using send_fd. This simplifies life for the client because
> >they also don't have to worry about cleanup using close_fd if the command
> >using the FD fails.
> 
> How do we identify file descriptors and then map them to a command?

IIUC, the FDs sent/received via struct cmsghdr are in a strictly
ordered array, so why not just define a placeholder syntax for
the commands that maps to the array indexes. eg

  netdev_add tap,fd=$0,vhost_fd=$1,id=hostnet0 

The '$' sign is not valid for a normal FD number, so use of a $0,
$1, $2, etc can reliably be substituted with the real FD number from
the cmsghdr array elements 0, 1, 2, etc

Regards,
Daniel
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