Re: BookE "branch taken" behavior vis-a-vis updating the NIP register

[Sakthi <vasanth.sri7@gmail.com>]

pegasus,

Were you able to achieve the intended behavior with any of the suggested RFC
patch?

Thanks!


pegasus wrote
> I re-read the link you posted earlier and this time it made more sense to
> me. The kind of questions which are coming into my mind were being
> discussed.
> 
> So, off I went and downloaded the latest version of
> arch/powerpc/kernel/traps.c hoping to see those very changes in them.
> However it didn't match one on one with what was written in that thread.
> Ditto for the other files in your patch. Looks like your patch didn't make
> it to upstream but it looks exactly like what I need here. So allow me to
> discuss certain finer points of it, to make sure I understand what it does
> correctly.
> 
> In that thread you say 
> James Yang wrote
>> BookE ISA's branch taken exception triggers before a branch that will be
>> taken executes.  This allows software to examine the branch and the
>> conditions under which it will be taken.  It also means software can tell
>> where basic blocks end (at least the ones which are terminated by taken
>> branches).  
*
>> There are no architected registers that report the address of the branch
>> instruction after it has executed.
*
> 
> My thoughts exactly! 
> 
> In the first patch's description, you say 
> James Yang wrote
>> This patch makes available the unmodified BookE branch taken debug
>> exception through PTRACE_SINGLEBLOCK if the ptrace() addr parameter is
>> set to 2.  (The existing behavior of PTRACE_SINGLEBLOCK is retained for
>> any other addr parameter value, e.g., 0.)  
*
>> SIGTRAP will be signaled with the NIP pointing to the branch instruction
>> before it has executed.  The ptrace-calling program can then examine the
>> program state.
*
>>   
/
>> It should then request a PTRACE_SINGLESTEP in order to advance the
>> program to the next instruction or a PTRACE_CONT to resume normal program
>> execution.
/
>>  The si_code now also reports TRAP_BRANCH.
>  So requesting PTRACE_CONT has to happen inside the SIGTRAP signal handler
> right? So as to advance the branch instruction (and since we are talking
> BookE here, we are dead sure this branch will be taken). Now as for the
> second patch, as far as I can see, implements the same functionality.
> However it makes the change permanent and any tool which is used to the
> NIP pointing to the branch target will be broken. 
> 
> Anyways, for me either of them will work. But I think the first patch
> makes everyone happy by using the 'addr' field of ptrace. This also means
> I will have to make my (broken) ptrace working which, it seems is not as
> easy adding an enum field as you suggested. May be theres a check
> somewhere in the actual ptrace code which checks for illegal values and
> hence even after adding an enum, it is being reported as illegal in my
> case. However getting that to work is another story.
> 
> Please confirm my understanding of your patches and since these patches
> have not made their way to the upstream kernel, will have to use them
> myself directly. By the way, I'm using 2.6.32.10 (you know..the long-term
> kernel) and I couldn't find any of your changes in them but then again I
> couldn't find it in the latest 3.12 version either.





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