[OpenBIOS] sparc32 trap recovery

[Mark Cave-Ayland]

On 15/06/13 18:50, Bob Breuer wrote:

> I'm aiming for a cpeek that catches the data faults on sparc32, so I've
> coded up the attached test code.
>
> The fault handler here is almost trivially simple: set a new instruction
> pointer and return to it.  All the magic is done in push_trap_handler
> and do_trap_handler, and they act similar to setjmp/longjmp.  Nesting is
> allowed, so it should be possible to use this at the top level for
> printing out pretty error messages for any fault just like OBP.
>
> I'm looking for any comments and suggestions.  This proof of concept
> only handles mmu data faults, but is extendable.  Example tests:
>    deadbeef try-fault
> returns false (0),
>    10000 try-fault
> returns true (-1) and the value at 0x10000
>
> Bob

I took a peek at the OpenBOOT source code to see what that does, and it 
seems that under the hood it is based on a trap handling solution. The 
key is newguarded-execute which on SPARC64 does the following:

   * Save the current contents of traps 30, 1f, 32 and 34
       0x30 = Data Access Exception
       0x1f = Level 15 interrupt
       0x32 = Data Access Error
       0x34 = Unaligned Access Error

   * Invoke the Forth word, throwing an a Forth exception upon error

   * Restore the contents of the above traps

 From the SPARCv8 manual we could say that these map to the following 
SPARC32 traps:

   * 0x09 Data Access Exception
   * 0x1f Level 15 interrupt
   * 0x29 Data Access Error
   * 0x07 Unaligned Access Error

In terms of the routine itself, have you had a look at the work I did a 
while back to catch the I/D-MMU miss traps on SPARC64? In particular I 
added SAVE_CPU_STATE(x) and RESTORE_CPU_STATE(x) macros to 
arch/sparc64/vectors.S that do all of the context save/restore work so 
that the code required to get back into C outside of this is fairly 
minimal (see reload_DMMU_tlb in the same file).


ATB,

Mark.