RAS trace event proto

[Borislav Petkov]

Hi all,

here's a dirty patch which should hopefully show what I have in mind wrt
using tracepoints for RAS events. The code compiles and should only give
an initial idea, it is subject to significant changes until it reaches
its final form thus it is only exemplary and not final in any case.

Notes:

* So there are two RAS tracepoints: trace_mce_record which dumps the
MCE-related errors + their decoded info and trace_hw_error which simply
carries a string to userspace. The second one can be used for non-MCA
errors.

* When prepping the string for the tracepoint, we cache the string by
calling ras_printk which buffers the so-far done string internally,
so everything that wants to dump into it needs to be converted to use
ras_printk.

* Which brings me to it: ras_printk() is x86-only and it could probably
be moved to an arch-agnostic place for the other arches. I'd leave it
x86-only for now, for testing purposes, and then later the other arches
could consider using it (this is wrt non-x86 EDAC drivers).

* When writing a 1 into /sys/devices/system/ras/agent, we enable the
string buffering functionality - this could be done by the RAS daemon or
whatever agent is requesting putting hw errors info into tracing.

* I'd like to have conditional printk-ing in trace_mce_record depending
on the TP args, Steve probably knows what can be done:

@Steven:

I'd like to do the following:

	TP_printk("%s, ARG1: %d, ARG2: %c ...", str1, arg1, arg2)

and have it print only the first arg, i.e. the string and drop the rest
of the args while still doing the TP_fast_assign into the ring buffer
and carrying the stuff to its consumers. Background is that I want to
dump the decoded string of a hardware error, if it is decoded, but carry
the MCE info to userspace and only dump the fields of the MCE if I
haven't managed to decode it, i.e. str1 == "".

So, my question is, can I do something like:

	TP_printk("%s, ARG1: %d, ARG2: %c ...", __print_conditional(str1, arg1, arg2))

where __print_conditional is a vararg macro which calls a
ftrace_print_cond() which prints only str1 if strlen(str1) > 0 and
otherwise calls a vsnprintf() variant to deal with the va_args?

As always, all comments are welcome.

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