The OVMF_CODE*.fd files provide UEFI firmware for a QEMU guest that is
intended to be read-only. The OVMF_VARS*.fd files provide UEFI variable
template images which are intended to be read-write, and therefore each
guest should be given its own copy. Here's an overview of each of them:

OVMF_CODE_4M.fd
  Use this for booting guests in non-Secure Boot mode. While this image
  technically supports Secure Boot, it does so without requiring SMM
  support from QEMU, so it is less secure. Use the OVMF_VARS.fd template
  with this.

OVMF_CODE_4M.secboot.fd
  Like OVMF_CODE_4M.fd, but will abort if QEMU does not support SMM.
  Use this for guests for which you may enable Secure Boot. If you specify
  this image, you'll get a guest that is Secure Boot-*capable*, but has
  Secure Boot disabled. To enable it, you'll need to manually import
  PK/KEK/DB keys and activate Secure Boot from the UEFI setup menu.

OVMF_VARS_4M.fd
  This is an empty variable store template, which means it has no
  built-in Secure Boot keys and Secure Boot is disabled. You can use
  it with any OVMF_CODE image, but keep in mind that if you want to
  boot in Secure Boot mode, you will have to enable it manually.

OVMF_VARS_4M.ms.fd
  This template has distribution-specific PK and KEK1 keys, and
  the default Microsoft keys in KEK/DB. It also has Secure Boot
  already activated. Using this with OVMF_CODE.ms.fd will boot a
  guest directly in Secure Boot mode.

OVMF32_CODE_4M.secboot.fd
OVMF32_VARS_4M.fd
  These images are the same as their "OVMF" variants, but for 32-bit guests.

OVMF_CODE.fd
OVMF_CODE.ms.fd
OVMF_CODE.secboot.fd
OVMF_VARS.fd
OVMF_VARS.ms.fd
  These images are the same as their "4M" variants, but for use with guests
  using a 2MB flash device. 2MB flash is no longer considered sufficient for
  use with Secure Boot. This is provided only for backwards compatibility.

OVMF_CODE_4M.snakeoil.fd
OVMF_VARS_4M.snakeoil.fd
  This image is **for testing purposes only**. It includes an insecure
  "snakeoil" key in PK, KEK & DB. The private key and cert are also
  shipped in this package as well, so that testers can easily sign
  binaries that will be considered valid.

PkKek-1-snakeoil.key
PkKek-1-snakeoil.pem
  The private key and certificate for the snakeoil key. Use these
  to sign binaries that can be verified by the key in the
  OVMF_VARS.snakeoil.fd template. The password for the key is
  'snakeoil'.

 -- dann frazier <dannf@debian.org>, Thu, 30 Sep 2021 10:33:08 -0600