Subject: Re: [PATCH -tip tracing/kprobes] PPC: Powerpc port of the kprobe-based event tracer

From: Michael Neuling <[email-masked]>
Date: Mon, 21 Dec 2009 07:54:04 +1100

In message <[email-masked]> you wrote:
> 
> 
> Mahesh Jagannath Salgaonkar wrote:
> > Michael Neuling wrote:
> >> In message <[email-masked]> you wrote:
> >>> Hi Michael,
> >>>
> >>> Michael Neuling wrote:
> >>>>> + * regs_get_argument_nth() - get Nth argument at function call
> >>>>> + * @regs:    pt_regs which contains registers at function entry.
> >>>>> + * @n:        argument number.
> >>>>> + *
> >>>>> + * regs_get_argument_nth() returns @n th argument of a function call.
> >>>>> + * Since usually the kernel stack will be changed right after
> >>>>> function en
> >> try
> >>>> ,
> >>>>> + * you must use this at function entry. If the @n th entry is NOT
> >>>>> in the
> >>>>> + * kernel stack or pt_regs, this returns 0.
> >>>>> + */
> >>>>> +unsigned long regs_get_argument_nth(struct pt_regs *regs, unsigned
> >>>>> int n)
> >>>>> +{
> >>>>> +    if (n < ARRAY_SIZE(arg_offs_table))
> >>>>> +        return *(unsigned long *)((char *)regs + arg_offs_table[n]);
> >>>>> +    else {
> >>>>> +        /*
> >>>>> +         * If more arguments are passed that can be stored in
> >>>>> +         * registers, the remaining arguments are stored in the
> >>>>> +         * parameter save area located at fixed offset from stack
> >>>>> +         * pointer.
> >>>>> +         * Following the PowerPC ABI, the first few arguments are
> >>>>> +         * actually passed in registers (r3-r10), with equivalent
> >>>>> space
> >>>>> +         * left unused in the parameter save area.
> >>>>> +         */
> >>>>> +        n += (PARAMETER_SAVE_AREA_OFFSET / sizeof(unsigned long));
> >>>>> +        return regs_get_kernel_stack_nth(regs, n);
> >>>> How do we handle FP args?
> >>> Currently this patch does not support FP args.
> >>
> >> This might be OK.  I don't think we use floating point parameters in any
> >> function definitions in the kernel. 
> >> We do use altivec in the raid6 driver (drivers/md/raid6altivec.uc) but
> >> they are static inline, so they probably don't even end up as
> >> functions. 
> >> I guess we need to make sure that we're not limiting the interface in
> >> such a way that we can't support it later if the above changes. 
> >> regs_get_argument_nth returns an unsigned long which makes returning a
> >> 128 bit VMX register impossible.  This might be a show stopper for me.
> >> How are the x86 guys dealing with this?
> >>
> > Nope, x86 does not deal with bigger registers (Masami, correct me if I
> > am wrong). The return data type is opaque to user. Hence this enables us
> > to handle any such situations in future without effecting user space API.
> 
> Right, we don't use those bigger registers in the kernel context.
> (some special functions use it inside, but most of codes
>  are just using general regs)

Sure, but kernel interfaces are for now.  What happens if this changes in
the future?

> And regs_get_argument_nth is just an accessor of pt_regs field.
> This means that it just provides field in pt_regs.

Sure, that's how it's implemented, but that's not what the name of the
function suggests it does.

Mikey

> 
> >>>>> +    }
> >>>>> +}
> >>>>> +/*
> >>>>>   * does not yet catch signals sent when the child dies.
> >>>>>   * in exit.c or in signal.c.
> >>>>>   */
> >>>>> Index: linux-2.6-tip/kernel/trace/Kconfig
> >>>>> ===================================================================
> >>>>> --- linux-2.6-tip.orig/kernel/trace/Kconfig
> >>>>> +++ linux-2.6-tip/kernel/trace/Kconfig
> >>>>> @@ -464,7 +464,7 @@ config BLK_DEV_IO_TRACE
> >>>>>  
> >>>>>  config KPROBE_EVENT
> >>>>>      depends on KPROBES
> >>>>> -    depends on X86
> >>>>> +    depends on X86 || PPC
> >>>>>      bool "Enable kprobes-based dynamic events"
> >>>>>      select TRACING
> >>>>>      default y
> >>>>>
> >>>>> _______________________________________________
> >>>>> Linuxppc-dev mailing list
> >>>>> [email-masked]
> >>>>> https://lists.ozlabs.org/listinfo/linuxppc-dev
> >>>>>
> >>> Thanks for reviewing.
> >>
> >> We are creating a new user space API here, so I'm keen for others to take
> >> a good look at the interface before we commit to something we are going
> >> to have to keep forever. 
> >> Who is the main consumer of this (/me is pretty ignorant of kprobes)?
> >> What do they think of the interface?
> >>
> > The user space API are already present in the upstream kernel and
> > currently only supported architecture is x86. This patch provides ppc
> > architecture specific interfaces that enables powerpc also in par with x86.
> 
> Yes, there is a kprobe tracer in ftrace (see Documentation/trace/kprobetrace.
txt).
> and this tracer is only for probing kernel (not userspace).
> 
> > 
> > The main consumer would be kernel developers who would like to see
> > register values, arguments and stack when the probe hits at given text
> > address.
> 
> Right.
> 
> BTW, there is a user-space tools we have (tools/perf/builtin-probe.c).
> Currently, it's only for x86. Mahesh, You just need to add a register
> translation table in tools/perf/util/probe-finder.c for ppc support.
> 
> Thank you!
> 
> -- 
> Masami Hiramatsu
> 
> Software Engineer
> Hitachi Computer Products (America), Inc.
> Software Solutions Division
> 
> e-mail: [email-masked]
>