Re: [Qemu-devel] [PATCH 0/2] [RFC] qemu-ga: add support for guest command execution

[Daniel P. Berrange]

On Tue, Dec 06, 2011 at 08:34:06AM -0600, Michael Roth wrote:
> The code is still in rough shape, but while we're on the topic of guest agents
> I wanted to put out a working example of how exec functionality can be added
> to qemu-ga to provide a mechansim for building arbitrarilly high-level
> interfaces.
> 
> The hope is that by allowing qemu-ga to execute commands in the guest, paired
> with file read/write access, we can instrument a guest "on the fly" to support
> any type of hyperviser functionality, and do so without dramatically enlarging
> the role qemu-ga plays as a small, QEMU-specific agent that is tightly
> integrated with QEMU/QMP/libvirt.
> 
> These patches add the following interfaces:
> 
> guest-file-open-pipe
> guest-exec
> guest-exec-status
> 
> The guest-file-open-pipe interface is analagous to the existing 
> guest-file-open
> interface (it might be best to roll it into it actually): it returns a handle
> that can be handled via the existing guest-file-{read,write,flush,close}
> interface. Internally it creates a FIFO pair that we can use to associate
> handles to the stdin/stdout/stderr of a guest-exec spawned process. We can 
> also
> also use them to redirect output into other processes, giving us the basic
> tools to build a basic shell (or a full-blown one if we add TTY support) using
> a single qemu-ga.
> 
> Theoretically we can even deploy other agents, including session-level agents,
> and communicate with them via these same handles. Thus, ovirt could deploy and
> run an agent via qemu-ga, Spice could deploy vdagent, etc. Since the interface
> is somewhat tedious, I'm working on a wrapper script to try out some of
> these scenarios, but a basic use case using the raw QMP interface is included
> below.
> 
> Any thoughts/comments on this approach are appreciated.
> 
> EXAMPLE USAGE (execute `top -b -n1`):
> 
> {'execute': 'guest-file-open-pipe'}
> {'return': 6}
> 
> {'execute': 'guest-exec',                    \
>  'arguments': {'detach': True,               \
>                'handle_stdout': 6,           \
>                'params': [{'param': '-b'},   \
>                           {'param': '-n1'}], \
>                'path': 'top'}}

This feels like a rather verbose way of specifying
the ARGV. Why not just allow

  {'execute': 'guest-exec',                    \
   'arguments': {'detach': True,               \
                 'handle_stdout': 6,           \
                 'params': ['-b', '-n1'],      \
                 'path': 'top'}}

Or even


  {'execute': 'guest-exec',                    \
   'arguments': {'detach': True,               \
                 'handle_stdout': 6,           \
                 'argv': ['top', '-b', '-n1']}} \

and just use the first element of argv as the binary to
execute. Also you might need to set env variables for
some tools, so we'd want

  {'execute': 'guest-exec',                    \
   'arguments': {'detach': True,               \
                 'handle_stdout': 6,           \
                 'argv': ['top', '-b', '-n1'], \
                 'env' : ['TMPDIR=/wibble']}}

and perhaps also you might want to run as a non-root
user, so allow a username/groupname ?

Regards,
Daniel
-- 
|: http://berrange.com      -o-    http://www.flickr.com/photos/dberrange/ :|
|: http://libvirt.org              -o-             http://virt-manager.org :|
|: http://autobuild.org       -o-         http://search.cpan.org/~danberr/ :|
|: http://entangle-photo.org       -o-       http://live.gnome.org/gtk-vnc :|