Implement CPU extensions in a kernel module

Question

Hi all,

I'm looking for information relating to implementing certain CPU extensions in a kernel module. I've found something related: http://www.mirrors.docunext.com/lxr/http/source/arch/mips/kernel/unaligned.c in fact, it's the only source code that I can find that is even close.

Basically, I have a binary only shared object built with certain CPU extensions, which I need to run on a slightly older CPU which has most of the instruction set, but not the fancy new stuff. Yeah, I know it'll be rather slow, but it's better than crashing with SIGILLs.

Thanks, j

Answer 1

i don't think you can fix this problem using a kernel module. i think you either need to run this in a VM which allows the missing instructions (I would try using XEN) or recompile the object so that it doesn't use them.

Answer 2

I think you can do this in userland. Install a handler for SIGILL with sigaction() and specify SA_SIGINFO. The field si_code in the siginfo_t allows distinguishing between several causes of SIGILL. For example, trying to emulate an instruction when the signal came from kill() does not make sense. The third argument to the handler points to a structure containing the CPU context at the time of the fault (see documentation). You can likely modify this and return from the signal handler, the changes taking effect; if that does not work, try setcontext().

Obviously, it will be a bit less efficient than doing it in the kernel, but cleaner and safer.