Re: [PATCH v2 3/3] tools/xl: Allow specifying JSON for domain configuration file format

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

On Tue, May 31, 2022 at 06:25:13PM -0700, Elliott Mitchell wrote:
> On Fri, May 20, 2022 at 03:12:46PM +0100, Anthony PERARD wrote:
> > On Tue, Apr 19, 2022 at 06:23:41PM -0700, Elliott Mitchell wrote:
> > > JSON is currently used when saving domains to mass storage.  Being able
> > > to use JSON as the normal input to `xl create` has potential to be
> > > valuable.  Add the functionality.
> > > 
> > > Move the memset() earlier so to allow use of the structure sooner.
> > > 
> > > Signed-off-by: Elliott Mitchell <ehem+xen@xxxxxxx>
> > 
> > So, I gave this a try and creating a guest from a json config, and that
> > fails very early with "Unknown guest type".
> > 
> > Have you actually tried to create a guest from config file written in
> > json?
> > 
> > Also, this would need documentation about the new option, about the
> > format. The man page need to be edited.
> > 
> > An example of a config file written in json would be nice as well.
> 
> I'll be trying for these at some point, but no timeframe yet.  This was
> an idea which occurred to me when looking at things.  I'm wavering on
> whether this is the way to go...
> 
> Real goal is I would like to generate a replacement for the `xendomains`
> init script.  While functional the script is woefully inadaquate for
> anything other than the tiniest installation.
> 
> Notably there can be ordering constraints for start/shutdown (worse,
> those could be distinct).  One might also wish different strategies for
> different domains (some get saved to disk on reboot, some might get
> shutdown/restarted).

Is this that something like `libvirt` or some other toolstacks could help
with? Or maybe you are looking for something that as a small footprint
on the system and just run once at boot and at shutdown.

> For some of the configuration for this, adding to domain.cfg files makes
> sense.  This though ends up with the issue of what should the extra data
> look like?

Maybe `xl` could be taught to ignore some options that have a prefix
like "xendomain_", even if at the moment it ignore everything it doesn't
know I think.

> I'm oscillating between adding a section in something libxl's parser
> takes as a comment, versus adding a configuration option to domain.cfg

A comment section could work too I guess, like there's one for `sysv`
init system which describe dependency as comments.

> (libxl's parser ignores unknown sections which is not entirely good!).
> JSON's structure would be good for an addition, but JSON comes with its
> own downsides.
> 
> Most likely such a thing would be implemented in Python.  Needs a bit
> more math than shell is good for.

If you plan to convert the `xendomains` init script to python, I don't
think that would be a good idea, as it is probably better to not add a
dependency for a init script that has been a shell script for a while.
But introducing a new utility in python or other language might be fine.

Cheers,

-- 
Anthony PERARD