Exposing -fw_cfg?

[Michal Privoznik]

List,

QEMU has capability to inject various blobs into firmware that configure 
how firmware configures itself. However, it can be also used to 
passthrough a specific file into the guest. For instance:

   -fw_cfg value=name=opt/com.example,file=/tmp/ign

will make the /tmp/ign file accessible in the guest under:

   /sys/firmware/qemu_fw_cfg/by_name/opt/com.example/raw

the @name is important here as it defines what knob is touched. For 
instance, /opt/ovmf tweaks OVMF, /bootorder changes the boot order, and 
so on. But, if the @name is in /opt/reverse.fully.qualified.domain form 
than this is a blob that is exposed into the guest and does not affect 
ACPI, SMBIOS, ... And IMO this is what makes the interface horrible.

While I definitely would not expose the FW configuration knobs (we 
already provide a way to configure things like bootroder), the file 
passthrough is actually used. So far I have found out that RHCOS uses it 
to give the guest so called ignition file (for the sake of argument we 
can assume it's like a kickstart that the OS reads on the first boot and 
configures itself up), but there are some other potential users (for 
users it looks intriguing, it's a simple API that makes a file show up 
at well defined location inside the guest).

Therefore I vouch for exposing the file passthorugh (and definitely do 
not mention firmware in the element or docs in any way, to not encourage 
users to use FW tweaking mode). However, before I design something, I'd 
like to hear your opinion.

Michal