Re: [patch 0/4] [RFC] mcount address adjustment

[Rabin Vincent <rabin@rab.in>]

On Thu, May 12, 2011 at 14:54, Martin Schwidefsky
<schwidefsky@de.ibm.com> wrote:
> On Wed, 11 May 2011 22:53:55 +0530 Rabin Vincent <rabin@rab.in> wrote:
>> On Tue, May 10, 2011 at 13:40, Martin Schwidefsky <schwidefsky@de.ibm.com> wrote:
>> Thumb-2 via recordmcount.pl needs the clearing of the lsb because the
>> relocation (R_ARM_ABS32) that gets used for the assembly file
>> that recordmcount.pl generates and assembles dictates that the lsb be
>> set if the target symbol is Thumb/Thumb-2 function.  mcount_adjust would
>> not help here since the ORing is done later, when the relocation is
>> applied.
>
> Hmm, from what I can make out the C version of recordmcount uses R_ARM_ABS32
> as well.

Right.  It worked when I initially implemented ARM support there because
recordmcount.c always found the STT_SECTION symbol as a base and not a
STT_FUNC symbol.  However, I noticed yesterday that this does not happen
in some cases, so I sent a patch to avoid STT_FUNC symbol as bases on
ARM, not because of this relocation, but because of a slightly different
oddity of Thumb symbols:

http://lkml.org/lkml/2011/5/11/304

(The relocation problem alone could be solved by using R_ARM_ABS32_NOI
 instead.)

>
>> Thumb-2 via recordmcount.c does not need the clearing of the lsb in
>> ftrace_call_adjust.
>
> So the clearing of the lsb is only required if the recordmcount.pl script
> is used?

Yes.

>> Building with the ARM instruction set also does not need the clearing
>> of the lsb.
>
> Who does the ORing? I can't find anything in recordmount.pl/recordmcount.c
> which looks like doing an OR, does the assembler do that based on the
> symbol type?

The lsb is set to 1 by the linker, when it applies the relocations as it
links vmlinux.

>
>> > Thumb-2 the offset is -1, correct? If there is a way to distinguish
>> > the two targets in recordmcount at compile time we could convert arm
>> > as well. Which would allow us to remove the ftrace_call_adjust function.
>>
>> To remove ftrace_call_adjust, we could either deprecate the
>> recordmcount.pl usage for ARM (you already have to edit the Kconfig to
>> use it) or modify it to generate specific relocations explicitly instead
>> of using the assembler data directives.
>
> Hmm, it would be a desirable property if the C version and the pearl
> version of recordmcount would do the same. Or we could remove the arm
> support from the pearl script, the C version is faster anyway.

I'm OK with removing the ARM support from recordmcount.pl; it doesn't
seem needed to make significant modifications to it for ARM when we
don't use it anyway.