Re: converting SWIs to pass swi number in a register.

[Matt Thomas <matt%3am-software.com@localhost>]

On Jul 2, 2008, at 2:43 PM, Chris Gilbert wrote:

> 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.

I'm already doing that for the THUMB SWI where syscall # > 255.
One question is how do we know we are using ip?  Think binary
compat with older binaries...