[PATCH] score/i386: Add context switch restore external symbol as a marker.

[Sebastian Huber]

On 22/07/16 08:34, Chris Johns wrote:
> On 22/07/2016 16:10, Sebastian Huber wrote:
>> On i386 the PC is stored on the stack frame:
>>
>> #define _CPU_Context_Initialize( _the_context, _stack_base, _size, \
>>                                     _isr, _entry_point, _is_fp,
>> _tls_area ) \
>>    do { \
>>      uint32_t   _stack; \
>>      \
>>      (void) _is_fp; /* avoid warning for being unused */ \
>>      if ( (_isr) ) (_the_context)->eflags = CPU_EFLAGS_INTERRUPTS_OFF; \
>>      else          (_the_context)->eflags = CPU_EFLAGS_INTERRUPTS_ON; \
>>      \
>>      _stack  = ((uint32_t)(_stack_base)) + (_size); \
>>      _stack &= ~ (CPU_STACK_ALIGNMENT - 1); \
>>      _stack -= 2*sizeof(proc_ptr*); /* see above for why we need to do
>> this */ \
>>      *((proc_ptr *)(_stack)) = (_entry_point); \
>>      (_the_context)->ebp     = (void *) 0; \
>>      (_the_context)->esp     = (void *) _stack; \
>>    } while (0)
>>
>> The stack pointer is part of the context. If you retrieve the PC from
>> here it should work for all threads.
>
> All threads on the i386 arch or all threads on all archs?
>
> I would like to make this simpler for all archs. I do not mind if we 
> require something being added to each arch to do this but I would like 
> an interface somewhere and somehow. 

I don't think that the thread context reconstruction can be done in an 
architecture independent way. You have different register sets and 
different ways to store the PC. For example on x86 the PC is on the 
stack, on ARM you have Context_Control::register_lr, on PowerPC 
ppc_context::lr which is embedded in Context_Control due to cache line 
alignment, on SPARC you have Context_Control::o7, and so on.

I would rather provide a global function that provides the data for your 
consumer (libunwind?) with a common interface for all architectures and 
an architecture dependent implementation. The debugger could call this 
function.

-- 
Sebastian Huber, embedded brains GmbH

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