[OpenBIOS] [PATCH 2/4] ppc: add new context handler

[Mark Cave-Ayland]

On 04/05/16 19:40, Segher Boessenkool wrote:

> On Wed, May 04, 2016 at 07:30:03PM +0100, Mark Cave-Ayland wrote:
>> On 04/05/16 16:31, Segher Boessenkool wrote:
>>
>>>> Can you explain further? The v2 patch uses a single register marked as
>>>> clobber for mflr/mtlr and the volatile modifier for __context ensures
>>>> that the old value being written to the stack doesn't get optimised away
>>>> by the compiler. What else am I missing?
>>>
>>> A normal (ABI-following) function can clobber all of r9..r12, so that
>>> "bl" instruction can, too.  If the function it calls is special (so at
>>> least written as assembler), things are different of course.
> 
> [ r3..r12, I can't type ]
> 
>> Right, we're talking about this section of code in the caller:
>>
>>     asm __volatile__ ("mflr %%r9\n\t"
>>                       "stw %%r9, %0\n\t"
>>                       "bl __switch_context\n\t"
>>                       "lwz %%r9, %0\n\t"
>>                       "mtlr %%r9\n\t" : "=m" (lr) : "m" (lr) : "%r9" );
>>
>>
>> The mflr/mtlr are being used to preserve the original link register
>> before branching to the assembler routine at __switch_context which does
>> all the hard work.
> 
> And the bl clobbers at least r4,r5.

I'm sorry but I still don't understand? __switch_context loads a
completely new CPU state from the stack in order to switch context so
actually every single register gets clobbered. But we preserve the
original link register in a local variable to ensure that we can jump
back to where we were upon return.


ATB,

Mark.