When the kernel first boots we have to be able to handle the gcc
generated jalr, addui sequence until ftrace_init gets a chance to run
and change the sequence. At this point mcount just adjusts the stack
and returns. When ftrace_init runs, we convert the jalr/addui to nops.
Then whenever tracing is enabled we convert the first nop to a "jalr
mcount+8". The mcount+8 entry point skips the stack adjust.
On Thu, Jan 17, 2013 at 1:27 AM, Geert Uytterhoeven
> On Thu, Jan 17, 2013 at 12:43 AM, Al Cooper <firstname.lastname@example.org> wrote:
>> Part of the sequence is "addiu sp,sp,-8" in the delay slot after every
>> call to the trace routine "_mcount" (some legacy thing where 2 arguments
>> used to be pushed on the stack). The _mcount routine is expected to
>> adjust the sp by +8 before returning.
> So when not disabled, the original jalr and addiu will be there, so _mcount
> to adjust sp.
>> The problem is that when tracing is disabled for a function, the
>> "jalr _mcount" instruction is replaced with a nop, but the
>> "addiu sp,sp,-8" is still executed and the stack pointer is left
>> trashed. When frame pointers are enabled the problem is masked
>> because any access to the stack is done through the frame
>> pointer and the stack pointer is restored from the frame pointer when
>> the function returns.
>> This patch writes two nops starting at the address of the "jalr _mcount"
>> instruction whenever tracing is disabled. This means that the
>> "addiu sp,sp.-8" will be converted to a nop along with the "jalr".
> When disabled, there will be two nops.
>> This is SMP safe because the first time this happens is during
>> ftrace_init() which is before any other processor has been started.
>> Subsequent calls to enable/disable tracing when other CPUs ARE running
>> will still be safe because the enable will only change the first nop
>> to a "jalr" and the disable, while writing 2 nops, will only be changing
> When re-enabled, there will be a jalr and a nop, which differs from the
> case, so _mcount doesn't have to adjust sp?
>> @@ -69,7 +68,7 @@ NESTED(ftrace_caller, PT_SIZE, ra)
>> .globl _mcount
>> b ftrace_stub
>> - nop
>> + addiu sp,sp,8
>> lw t1, function_trace_stop
>> bnez t1, ftrace_stub
> But _mcount will always adjust the stack pointer?
> What am I missing?
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 --
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like
> -- Linus Torvalds