I'm curious if anyone else has tried to create a truly minimal MetaROUTER image.
"Truly minimal", in this context, would mean a non-Linux firmware image whose sole function would be to service the "VIRQ" virtual interrupts associated with the virtual interfaces (and optionally the virtual console).
Such an approach would yield tiny RAM usage and use less CPU time, allowing multiple Meta images even on tiny devices like the RB750. There doesn't seem much point in running many megabytes of virtualized Linux code when all that is needed is packet processing.
The barriers to this seem to be trying to infer the VIRQ interface API and relevant hardware-specific initialization by deciphering the Mikrotik patches to Linux. I'm not aware of any formal documentation, and I presume that it might change over time.
For anyone else interested, it seems like the best results can be achieved by searching the patched Linux kernel source code for "VIRQ_BASE" and "get_virq_nr". (Different code references VIRQ differently.) For the hardware-specific initialization, it is hard to tell what is relevant and what is just specific to features needed for Linux.
A MetaROUTER image appears to just be a .tgz file containing an uncompressed .elf file of the kernel plus any file system files.
Given this, a "truly minimal" image would just be a .tgz file containing an .elf file named "kernel" with the raw VIRQ service routine MIPS code.