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

[Luiz Capitulino <lcapitulino@redhat.com>]

On Thu, 11 Nov 2010 16:39:52 +0000
"Daniel P. Berrange" <berrange@redhat.com> wrote:

> 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

I have the impression that the real problem is that they are being
stored in the wrong place (ie. the monitor), *maybe* they should be stored
somewhere else, ie. a special chardev or something.

Today those fds are linked to a given monitor session..