Re: [RFC PATCH v2] perf/x86: make perf callchain work without CONFIG_FRAME_POINTER

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Apr 16, 2019 at 03:15:59PM -0500, Josh Poimboeuf wrote:
> If you do the regs->eflags thing to mark the regs as fake in
> (perf_arch_fetch_caller_regs()), then I don't think skip_sp would be
> needed, because regs->sp could probably mark the skip point.
> 
> Instead I was actually hoping we could get rid of fake regs and
> perf_arch_fetch_caller_regs() altogether, because it's a nasty hack.
> But I don't know what else those fake regs are used for.

This is the generic perf generate a sample path. It doesn't know the
context. Normally it is an interrupt or exception of sorts and we simply
pass the pt_regs from that down the chain and all is good.

It is just for a few software events, such as SW_CONTEXT_SWITCH,
SW_MIGRATIONS and all the TP muck that we do not have regs to pass down.

These regs are used by:

 - SAMPLE_IP,
 - SAMPLE_CALLCHAIN (this here issue),
 - SAMPLE_REGS_INTR,
 - a few misc bits

There's actually comment on top of perf_fetch_caller_regs() that tries
to explain what is needed -- although I suppose that could do with an
update.