[RFC PATCH] powerpc/64/ftrace: mprofile-kernel patch out mflr

[Naveen N. Rao]

Michael Ellerman wrote:
> Nicholas Piggin <npiggin at gmail.com> writes:
>> The new mprofile-kernel mcount sequence is
>>
>>   mflr	r0
>>   bl	_mcount
>>
>> Dynamic ftrace patches the branch instruction with a noop, but leaves
>> the mflr. mflr is executed by the branch unit that can only execute one
>> per cycle on POWER9 and shared with branches, so it would be nice to
>> avoid it where possible.
>>
>> This patch is a hacky proof of concept to nop out the mflr. Can we do
>> this or are there races or other issues with it?
> 
> There's a race, isn't there?
> 
> We have a function foo which currently has tracing disabled, so the mflr
> and bl are nop'ed out.
> 
>   CPU 0			CPU 1
>   ==================================
>   bl foo
>   nop (ie. not mflr)
>   -> interrupt
>   something else	enable tracing for foo
>   ...			patch mflr and branch
>   <- rfi
>   bl _mcount
> 
> So we end up in _mcount() but with r0 not populated.

Good catch! Looks like we need to patch the mflr with a "b +8" similar 
to what we do in __ftrace_make_nop().


- Naveen