Re: [RFC PATCH v3] perf/x86: make perf callchain work without CONFIG_FRAME_POINTER

From: Kairui Song
Date: Fri Apr 19 2019 - 15:11:07 EST

On Fri, Apr 19, 2019 at 5:43 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Fri, Apr 19, 2019 at 10:17:49AM +0800, Kairui Song wrote:
> > On Fri, Apr 19, 2019 at 8:58 AM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> > >
> > > I still don't like using regs->bp because it results in different code
> > > paths for FP and ORC. In the FP case, the regs are treated like real
> > > regs even though they're fake.
> > >
> > > Something like the below would be much simpler. Would this work? I don't
> > > know if any other code relies on the fake regs->bp or regs->sp.
> >
> > Works perfectly. My only concern is that FP path used to work very
> > well, not sure it's a good idea to change it, and this may bring some
> > extra overhead for FP path.
> Given Josh wrote all that code, I'm fairly sure it is still OK :-)
> But also looking at the code in unwind_frame.c, __unwind_start() seems
> to pretty much do what the removed caller_frame_pointer() did (when
> .regs=NULL) but better.

OK, with FP we will also need to do a few more extra unwinding,
previously it start directly from the frame of the trace point, now
have to trace back to the trace point first.
If that's fine I could post another update (that will be pretty much
just copy&paste from the Josh's code he posted :P , is this OK?)

Best Regards,
Kairui Song