[OpenBIOS] [RFC 3/3] ppc: RTAS WIP

[Alexander Graf]

On 15.10.2010, at 22:56, Andreas Färber wrote:

> Am 15.10.2010 um 22:28 schrieb Alexander Graf:
> 
>> On 15.10.2010, at 22:22, Andreas Färber wrote:
>> 
>>> Am 15.10.2010 um 12:26 schrieb Alexander Graf:
>>> 
>>>> On 15.10.2010, at 00:38, Alexander Graf wrote:
>>>> 
>>>>> /* saving non-volatile registers */
>>>>> 
>>>>>> +    stw r13,  8(r4)
>>>>> 
>>>>> I would recommend multiplying here:
>>>>> 
>>>>> stw r13, (3 * 4)(r4)
>>>>> 
>>>>> That makes it more readable.
>>>>> 
>>>>>> +    stw r14, 12(r4)
>>>>>> +    stw r15, 16(r4)
>>>>>> +    stw r16, 20(r4)
>>>>>> +    stw r17, 24(r4)
>>>>>> +    stw r18, 28(r4)
>>>>>> +    stw r19, 32(r4)
>>>>>> +    stw r20, 36(r4)
>>>>>> +    stw r21, 40(r4)
>>>>>> +    stw r22, 44(r4)
>>>>>> +    stw r23, 48(r4)
>>>>>> +    stw r24, 52(r4)
>>>>>> +    stw r25, 56(r4)
>>>>>> +    stw r26, 60(r4)
>>>>>> +    stw r27, 64(r4)
>>>>>> +    stw r28, 68(r4)
>>>>>> +    stw r29, 72(r4)
>>>>>> +    stw r30, 76(r4)
>>>>>> +    stw r31, 80(r4)
>>>> 
>>>> Actually thinking about this a bit more, r13-r31 are already defined non-volatile by the C ABI, so you can be sure that the C function you're calling doesn't clobber them. You don't need to manually save/restore them :).
>>> 
>>> The OF client interface saves r5-r31 as well as ctr, cr and xer.
>>> It shouldn't hurt to save more registers than necessary to assert the requirements of the CHRP spec [1]:
>>> 
>>> <<<
>>> 7.2.2 Register Usage
>>> 
>>> Requirements:
>>> 
>>> 7–10. Except as required by a specific function, RTAS must not modify the following operating environment registers: TB, DEC, SPRG0-SPRG3, EAR, DABR, SDR1, ASR, SR0-SR15, FPSCR, FPR0-FPR3, and any processor specific registers.
>> 
>> Read: no timers, no paging, no modification of page fault handler registers
> 
>>> 7-11. RTAS must preserve the following user mode registers: R1-R2, R13-R31, and CR.
>> 
>> Except for CR, C is the same. So you really only need to save/restore cr :).
> 
> Huh? Doesn't that depend on the ABI used rather than on C? If someone uses a differently configured GCC (or clang or ...), such assumptions might not hold.

I thought we're using the Linux ABI internally?

> What about r3-r4? The SysV ABI ppc supplement has them listed as volatile, so if I want to continue to access the private data area, I still need to save and restore r4, no?

r3 and r4 are volatile, yes. But those are not listed in the paragraphs here either :).


Alex