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

[Andreas Färber]

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.

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?

Andreas

>> 7–12. RTAS must preserve the following operating environment  
>> registers: MSR, DAR, DSISR, IBAT0-IBAT3, and DBAT0-DBAT3.
>
> Read: You're allowed to change direct mappings, but need to restore  
> them when exiting again. Interesting :).
>
>
> Alex
>