corrected: tapset/script information template

[William Cohen]

Ananth N Mavinakayanahalli wrote:
> William Cohen wrote:
> 
>> Ananth N Mavinakayanahalli wrote:
>>
>>> Vara Prasad wrote:
>>>
>>>> Hi Will,
>>>>
>>>> Thanks for taking care of this.  This is an excellent start for 
>>>> function tracing.
>>>> I am sure this feature will make many kernel developers happy.
>>>>
>>>> Ananth, can you check to see if this approach works on PPC platform.
>>>>
>>>
>>> PowerPC (POWER4 and above) have hardware support only for data address
>>> breakproints (they have one Data Address Breakpoint Register - DABR). If
>>> we are talking of using debug hadware to track text addresses, then, no
>>> PPC doesn't provide a facility for us to use.
>>>
>>> IIUC, the PPC designers discontinued IABR (Instruction Address
>>> Breakpoint Register) since they thought all breakpoints on a text 
>>> address can be handled in software.
>>>
>>> Thanks,
>>> Ananth
>>
>>
>>
>> The 970FX manual mentions the IABR.
> 
> 
> Yes it does, but as you may know there are numerous variants of the
> Power processor and the pSeries line uses Power4 and above.
> 
> arch/ppc64/kernel/cputable.c in the kernel sources has a listing of the
> different types of Power processors and their capabilities. Power4 and
> above don't even have a IABR. But, I don't see the IABR being
> "supported" on Linux even for the 970(FX) as the ppc970(fx) struct
> doesn't list CPU_FTR_IABR.
> 
>> However, I was thinking about using branch trace mode. I am looking at 
>> page 116 of the 970FX_user_manual_v1.14.pdf. The Machine State 
>> Register (MSR) has bit 54 which appear to cause a trap for each 
>> branch. The main difficulty I see with PPC is distiguishing between a 
>> branch and return. Using this would allow tracing a particular thread. 
>> It won't be cheap for the tread being traced, but it won't incurr 
>> overhead on the threads not being traced.
>>
>> What approaches for handling breakpoints on text address were 
>> suggested? Replacing the instruction at that location with a software 
>> break point? That would be triggered on threads that encountered that 
>> location. The dynamic rewrite of the code?
> 
> 
> "Do it all in software" which would mean that it'd be triggered on
> threads that encountered that location. I will try and get in touch with
> the ppc Linux gurus and get more clarity on this.

Ananth,

Could you talk with the ppc linux guru about using the branch trap that 
is provided in ppc's MSR? See how well/poorly the branch traps are 
supported?

-Will