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SubjectRe: want to clarify powerpc assembly conventions in head.S and entry.S

Benjamin Herrenschmidt wrote:
>>You knew this was coming... What's special about syscalls? There's the
>>r3 thing, but other than that...
>
> The whole codepath is a bit different, there's the syscall trace,
> we can avoid saving much more registers are syscalls are function
> calls and so can clobber the non volatiles, etc...

It appears that we always enter the kernel via "transfer_to_handler",
and return via "ret_from_except". Is this true? (I'm running on at
least a 74xx chip.)

I want to insert two new bits of code, one that gets called before the
exception handler when we drop from userspace to kernelspace, and one as
late as possible before going back to userspace. I need to catch
syscalls, interrupts, exceptions, everything.

The entry one I planned on putting in "transfer_to_handler", just before
"addi r11,r1,STACK_FRAME_OVERHEAD".

I was planning on putting the exit one just after the "restore_user"
label. Will this catch all possible returns to userspace?

Thanks,

Chris

** Sent via the linuxppc-dev mail list. See http://lists.linuxppc.org/


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