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

["Denis V. Lunev" <den-lists@parallels.com>]

On 03/02/15 22:09, Gabriel L. Somlo wrote:
> Hi,
>
> 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
>
> So far, the environment would be read-only from the guest perspective.
> A nice additional feature would be to allow setting additional guest-env
> variables, or replacing values for existing ones:
>
>      $ qemu-guest-env set VAR3 "foo bar"
>
>      $ qemu-guest-env set VAR4 "blah"
>
> The actual QGA could then be taught to export guest-env set and get
> commands, so that querying/setting data in the guest environment could
> be allowed from the outside, via e.g. 'virsh qemu-agent-command'.
>
> The 'qemu-guest-env' functionality could be incorporated into 'qemu-ga',
> or left as a separate (but closely related) binary, most likely packaged
> together with qemu-ga.
>
> In terms of the mechanics, I'll need to figure out the following:
>
> 1. How would -guest-env="..." actually pass data into the guest?
>
>     Off the top of my head, we could piggyback on top of smbios (by using
>     a type 11, "OEM strings" structure), which could then be parsed by
>     qemu-guest-env from the guest side.
>
> 2. How to support "set-env" (in addition to "get-env") functionality?
>
>     set-env means access to a writable guest memory area, and the least
>     inconvenient way I can think of accomplishing this would be to put
>     QGA in charge of it. When starting up for the first time, QGA would
>     grab the host-supplied environment (e.g. by parsing smbios type 11),
>     then respond to subsequent get and set requests from its in-memory
>     environment key-value store. Changes to the environment made after
>     the guest was started would not be persistent, but that's not something
>     currently offered by any other platforms either :)
>
> Ultimately, if this makes it into QEMU, I'd like to also expose the
> functionality through libvirt/virsh/virt-manager (so users could set
> the initial environment variables as part of the guest VM xml template,
> via the gui, etc.)
>
> So, my question for the QEMU dev team:
>
> 1. Would you consider this feature a useful addition to QEMU ?
>     I.e., would this be acceptable (of interest) to the upstream project?
>
> 2. Is anything similar already being worked on (so I could either join
>     that effort, or back off, as the case may be) ? :)
>
> 3. Any technical advice on how to (better) implement it ?
>
> Thanks much,
> --Gabriel
>


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.

Unfortunately guest exec functionality is still pending...