Re: Add private syscalls to support NPTL

[Maxim Kuvyrkov <maxim@codesourcery.com>]

Brad Boyer wrote:
> On Tue, Aug 18, 2009 at 01:48:39AM +0400, Maxim Kuvyrkov wrote:
>> +/* This syscall gets its arguments in A0 (mem), A1 (oldval) and
>> +   D1 (newval).  */
>> +asmlinkage int
>> +m68k_sys_atomic_cmpxchg_32(unsigned long newval, int d2, int d3, int d4, int d5,
>> +			unsigned long __user *mem, unsigned long oldval)
>> +{
> 
> Any particular reason for this calling convention? The real CAS instruction
> specifically takes two data registers for the two values. It seems more
> logical to me to have oldval and newval in data registers and just have
> the mem argument in an address register.

The reason for this calling convention is to minimize difference in 
invokation of the vDSO helper and the syscall.  The helper gets its 
arguments in a0 (mem), d0 (oldval) and d1 (newval); in a stub, which is 
used when vDSO is not available, one then can do:

movel %d0,%a1
movel #NR,%d0
trap #0

--
Maxim