Onboarding QEMU Firmware Configuration to other machines

[Anne Macedo <retpolanne@posteo.net>]

Hello!

How should one port fw_cfg to other QEMU machines?

Context:

- I found a possible bug on the Debian builds for Raspberry Pi 4 that is
  only triggered when I try to launch it on QEMU;

- I wrote an issue on systemd (I thought it had something to do with
  logind) and they requested me to only fill bugs if they show up on the
  upstream.

- I tried baking systemd with mkosi – however, to test with mkosi and
  qemu, it uses fw_cfg [1] which seems specific to the virt machine on
  ARM.

- Opened an issue [2] and started working on that [3]

However, I still don't understand exactly how fw_cfg works. My current
understanding is that it is a device that, when connected some memory
mapping will happen on the guest so that it can access this memmap.

TL;DR which steps should I take to onboard fw_cfg to machines such as
raspi4b?

Thanks!
Anne


[1] https://github.com/systemd/mkosi/blob/6c33859a1b445e0bff19d9e7f28158140893a3e4/mkosi/config.py#L456

[2] https://gitlab.com/qemu-project/qemu/-/issues/2695

[3] https://gitlab.com/retpolanne/qemu/-/commit/86fca16555385c3685b7a1489a9cfaa255e148cf