For example, say we wanted to put a general call for sti into entry.S,
where its expected it won't touch any registers. In that case, we'd
have a sequence like:
push %eax
push %ecx
push %edx
call paravirt_cli
pop %edx
pop %ecx
pop %eax
If we parse the relocs, then we'd find the reference to paravirt_cli.
If we look at the byte before and see 0xe8, then we can see if its a
call. If we then work out in each direction and see matched push/pops,
then we know what registers can be trashed in the call. This also
allows us to determine the callsite size, and therefore how much space
we need for inlining.
