converting SWIs to pass swi number in a register.

[Chris Gilbert <chris%dokein.co.uk@localhost>]

Hi,

I was wondering if anyone had an opinion on switching to passing the SWI 
 number into syscall.  I know Linux did this a while back.

I think it particularly makes sense for processors with seperate 
instruction and data caches, as it save taking a data cache miss for 
every SWI call.

Looking at the current code and our current ABI, I'm thinking we could 
use ip (r12) to pass the SWI to the kernel, eg SYSTRAP becomes similar 
to this:
#define SYSTRAP(x)      \
                        mov r12, # ((SYS_ ## x) & 0xff) ;\
                        add r12, r12, #((SYS_ ## x) & 0xff00) ;\
                        swi SWI_OS_NETBSD | SYS_ ## x

Note I thinking we can probably a bit of tweaking to remove an 
instruction count if the swi is < 256

This would allow a post 5.0 kernel to only need to look at the r12/ip to 
work out the SWI call.

Thoughts?

Thanks,
Chris