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

[Naveen N. Rao]

Michael Ellerman wrote:
> "Naveen N. Rao" <naveen.n.rao at linux.ibm.com> writes:
>> 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().
> 
> Would that actually make it any faster though? Nick?

Ok, how about doing this as a 2-step process?
1. patch 'mflr r0' with a 'b +8'
   synchronize_rcu_tasks()
2. convert 'b +8' to a 'nop'

- Naveen