[Pkg-libvirt-maintainers] Bug#764834: split files

[Laszlo Ersek]

OVMF_VARS.fd and OVMF_CODE.fd are split files. OVMF.fd is a unified
file. Concatenating the first two yields the third.

OVMF_VARS.fd is a variable store template. OVMF_CODE.fd is the fimrware
executable. When you create a new VM, libvirt instantiates the VM's own,
private variable store by copying OVMF_VARS.fd. And, all VMs share the
OVMF_CODE.fd binary.

When you update the OVMF package, the varstore template and the
executable are both replaced, but the VMs preserve their own, private
variable stores. This was the reason for implementing the split files --
so that the firmware be possible to update centrally on a host, but the
VMs' varstores remain unaffected.

(If you use a unified file, then the varstore part of that file is not a
template, but a live variable store, and if you replace the entire file,
then the VM loses its variables. In addition, each VM would have a
separate copy of the executable part as well, which would require
touching each VM's copy when updating the firmware on a host system, to
get a central update.)

Regarding the place of OVMF_VARS.fd and OVMF_CODE.fd in the filesystem
-- you put them wherever you want in Debian I guess, but they *really*
correspond to each other. For example, a Secure Boot enabled
OVMF_CODE.fd will not work with varstores copied from an OVMF_VARS.fd
that was the output of a Secure Boot disabled build. IOW, OVMF_VARS.fd
is not a config file for the user to update, it's really the counterpart
of OVMF_CODE.fd.

https://github.com/tianocore/edk2/commit/1c50db8a

Debian's OVMF package should probably be updated, and the installed
files should include the split files (preferably: *only* the split files).