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

[Anthony Liguori]

On 11/11/2010 09:55 AM, Daniel P. Berrange wrote:
> On Thu, Nov 11, 2010 at 04:47:41PM +0100, Markus Armbruster wrote:
>    
> > Luiz Capitulino<address@hidden>  writes:
> >
> >      
> > > On Wed, 10 Nov 2010 14:20:12 +0100
> > > Markus Armbruster<address@hidden>  wrote:
> > >
> > >        
> > > > Luiz Capitulino<address@hidden>  writes:
> > > >          
> > [...]
> >      
> > > > > diff --git a/qmp-commands.hx b/qmp-commands.hx
> > > > > index 793cf1c..b344096 100644
> > > > > --- a/qmp-commands.hx
> > > > > +++ b/qmp-commands.hx
> > > > > @@ -761,6 +761,51 @@ Example:
> > > > >
> > > > >   Note: This command must be issued before issuing any other command.
> > > > >
> > > > > +EQMP
> > > > > +
> > > > > +    {
> > > > > +        .name       = "hmp_passthrough",
> > > > > +        .args_type  = "command-line:s,cpu-index:i?",
> > > > > +        .params     = "",
> > > > > +        .help       = "",
> > > > > +        .user_print = monitor_user_noop,
> > > > > +        .mhandler.cmd_new = do_hmp_passthrough,
> > > > > +    },
> > > > > +
> > > > > +SQMP
> > > > > +hmp_passthrough
> > > > > +---------------
> > > > > +
> > > > > +Execute a Human Monitor command.
> > > > > +
> > > > > +Arguments:
> > > > > +
> > > > > +- command-line: the command name and its arguments, just like the
> > > > > +                Human Monitor's shell (json-string)
> > > > > +- cpu-index: select the CPU number to be used by commands which access CPU
> > > > > +             data, like 'info registers'. The Monitor selects CPU 0 if this
> > > > > +             argument is not provided (json-int, optional)
> > > > > +
> > > > > +Example:
> > > > > +
> > > > > +->  { "execute": "hmp_passthrough", "arguments": { "command-line": "info kvm" 
> > > > > } }
> > > > > +<- { "return": "kvm support: enabled\r\n" }
> > > > > +
> > > > > +Notes:
> > > > > +
> > > > > +(1) The Human Monitor is NOT an stable interface, this means that command
> > > > > +    names, arguments and responses can change or be removed at ANY time.
> > > > > +    Applications that rely on long term stability guarantees should NOT
> > > > > +    use this command
> > > > > +
> > > > > +(2) Limitations:
> > > > > +
> > > > > +    o This command is stateless, this means that commands that depend
> > > > > +      on state information (such as getfd) might not work
> > > > > +
> > > > > +    o Commands that prompt the user for data (eg. 'cont' when the block
> > > > > +      device is encrypted) don't currently work
> > > > > +
> > > > >   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?

Regards,

Anthony Liguori

> Regards,
> Daniel
>