Re: SVC (SWI) for Thumb

[David Laight <david%l8s.co.uk@localhost>]

On Tue, Aug 13, 2013 at 06:30:14PM -0700, Matt Thomas wrote:
> 
> Currently, NetBSD/arm uses a "global" SVC space (see <arm/swi.h) but
> in Thumb mode, the SWI instruction can only hold an 8 bit value and
> we have nearly 512.  The 0x00a0000. indicated the NetBSD SVC space.
...
> So my thought is to reserve svc codes in thumb mode, 254 and 255.
> In either case, if the syscall number is less than 254, it's used 
> directly with svc.
> 
> 255 is for thumb on systems with thumb2 available and any syscall 
> number >= 254 is placed in the ip (r12) register via a movw
> instruction.  This is simple and straight forward.
> 
> 254 is for thumb on systems with just thumb1 and any system call
> number >= 254.  The argument in r0 is moved to ip since ip isn't
> easily used.  r0 will contain the syscall number - 254.  This
> allows for a single addition movs r0, #<syscall - 254> for calls
> less than 510.  For syscalls >= 512, this sequence is used.
> 
>       movs    r0, #(syscall - 254)/8
>       lsls    r0, r0, #3

Shouldn't that be a shift by 8 ?

>       adds    r0, r0, #((syscall - 254) & 7)
> 
> This allows up to 2047 syscalls which is plenty of room for growth.
> 
> The SWI handler will do the adjustments needed when it encounters
> a thumb SVC 254 or 255.

Why not just use r0 for thumb v1 and v2 ?

And why not pass the syscall number in a register for all syscalls?
Setting an 8 bit value costs one instruction in userspace, but probably
removes one from the kernel side (does it have to read the swi instruction
word to get the value?).

        David



        David

-- 
David Laight: david%l8s.co.uk@localhost