Re: [Qemu-devel] RFC: Proposal to add QEMU "Guest Environment Variables"

[Denis V. Lunev]

On 03/02/15 23:14, Gabriel L. Somlo wrote:
> Hi Denis,
>
> On Tue, Feb 03, 2015 at 10:14:03PM +0300, Denis V. Lunev wrote:
> > On 03/02/15 22:09, Gabriel L. Somlo wrote:
> > > I'm interested in adding a way for a host to pass environment variables
> > > into a qemu guest VM -- analogous to setting environment variables for
> > > a process to access via getenv() and friends.
> > >
> > > The QEMU Guest Agent (QGA) does not appear to quite fit the bill, at
> > > least not in its current form: The agent must have been successfully
> > > started on the guest before the host would have to connect to it (in
> > > a separate act from just starting the guest in the first place), and
> > > get it to execute any hypothetical commands to configure or otherwise
> > > influence the guest.
> > >
> > > For this functionality to really resemble the way environment variables
> > > are used, environment information should be provided as part of the QEMU
> > > command line, without the requirement to make a separate/subsequent
> > > connection to the guest agent. For example:
> > >
> > >      qemu-system-x86_64 -guest-env="VAR1=value1;VAR2=value 2" -hda image.qcow2
> > >
> > > Once the guest is started, it should be possible to query the guest
> > > environment with something like:
> > >
> > >      $ qemu-guest-env get VAR1
> > >      value1
> > >
> > >      $ qemu-guest-env get VAR2
> > >      value 2
> > > [...]
> >
> > I think that you can do this through guest write/guest exec commands.
> > You can configure service start to perform it is a proper way through
> > guest read/guest write and after that restart the agent.
>
> Are these supposed to be commands issued from the qemu monitor prompt
> (or qmp) ? My Google-fu didn't help too much either :)
>
> The main idea is to be able to provide environment data to a guest
> at start time (on the qemu command line), and not have to worry about
> connecting back to it (via monitor prompt, qmp, if/when it starts the
> guest agent, etc) to pass more information to it later on.
>
> I'm probably misunderstanding what you said, though, so please
> clarify...
>
> Thanks much,
> --Gabriel
QEMU guest agent starts as a service inside the guest. Its startup
is controlled through standard Unix scripts. You can modify these
scrips from the host side and (hopefully) restart agent.

The modification could be made using

##
# @GuestFileRead
#
# Result of guest agent file-read operation
#
# @count: number of bytes read (note: count is *before*
#         base64-encoding is applied)
#
# @buf-b64: base64-encoded bytes read
#
# @eof: whether EOF was encountered during read operation.
#
# Since: 0.15.0
##
{ 'type': 'GuestFileRead',
  'data': { 'count': 'int', 'buf-b64': 'str', 'eof': 'bool' } }

##
# @GuestFileWrite
#
# Result of guest agent file-write operation
#
# @count: number of bytes written (note: count is actual bytes
#         written, after base64-decoding of provided buffer)
#
# @eof: whether EOF was encountered during write operation.
#
# Since: 0.15.0
##
{ 'type': 'GuestFileWrite',
  'data': { 'count': 'int', 'eof': 'bool' } }

Den