[Buildroot] [PATCH 2/2] package/jailhouse: add option to choose custom repo/branch

[Yann E. MORIN <yann.morin.1998@free.fr>]

Mario, Ralf, All,

On 2020-06-01 18:23 +0200, Mario Mintel spake thusly:
> Am Sa., 30. Mai 2020 um 19:28?Uhr schrieb Mario Mintel < [1]mariomintel@gmail.com>:
>   Am Fr., 29. Mai 2020 um 23:40?Uhr schrieb Ralf Ramsauer < [2]ralf.ramsauer@oth-regensburg.de>:
>   On 5/29/20 10:48 PM, Yann E. MORIN wrote:
>   > Mario, All,
>   > On 2020-05-28 16:43 +0200, Mario Mintel spake thusly:
>   >> In addition to official releases of Jailhouse, allow to specify a custom
>   >> Git URI + branches. This adds more flexibility for custom
>   >> configurations.
>   > The overwhelming majority of packages do not allow selecting an
>   > alternate location. Why would jailhouse be different?
> 
>   Jailhouse requires system-specific configurations. Those configurations
>   are compiled from C source files to binaries during the build process.
>   While upstream Jailhouse comes with a lot of samples for supported
>   systems, you will need a lot of fine tuning to for a specific use case.

I am not sure I entirely followed... Note that I am totally ignorant to
how jailhouse works (and I barely know what it is). So I had a quick
look into the github repo, and I noticed this:

    A system configuration can be created on an x86 target system by
    running the following command:

        jailhouse config create sysconfig.c

    In order to translate this into the required binary form, place
    this file in the configs/x86/ directory. The build system will pick
    up every .c file from there and generate a corresponding .cell file.

Is this what you were trying to explain?

If so, then I think we need a way for people to indeed provide their
cells descriptions files, so that they do get compiled by jailhouse,
without resorting to using an OVERRDIE_SRCDIR.

So I see a few options:

 1- let people provide those .c files as a patch against jailhouse. This
    requires no infra in Buildroot, but this is not very convenient;

 2- add a configuration option in jailhouse/Config.in, which people
    could set as a path to a directory with .c files; those would be
    copied into the jailhouse build directory before the actual build,
    so the documented way (see above) will be used; those files would
    have to be in a br2-external tree or whatever, but not in a package
    (because we'd have no way to ensure that package be extracted before
    jailhouse gets built).

 3- let people write their own package(s) (e.g. in a br2-external tree)
    that only builds the cell files. That package would depend on
    jailhouse (or rather, the to-be-introduced host-jailhouse). And
    packages could also provide their own cell definitions, too...

I think option 3 is the best solution, as it is the most flexible and
most generic one. However, it will depend on the possibility to
introduce a host-jailhoue package that can just install the 'jailhouse
cell cross compiler'. As far as I see, this is just a bunch of objcopy,
but this is quite tightly integrated into the kernel Kbiuld process, so
might not so simple to come up with.

Option 2 is probably a good compromise if option 3 turns out to be too
difficult to come up with...

[--SNIP--]
> It does work as proposed by Yann. I wasn't aware of that option. I
> guess that makes this patch redundant.

Using a completely separate tree? Yes, I think this is not an option.

However, given the above, maybe we still need a way for people to
provide their cells descriptions and have them somehow copied at build
(or configure) time...

Regards,
Yann E. MORIN.

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