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Date:   Fri, 19 Apr 2019 11:43:13 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Kairui Song <kasong@redhat.com>
Cc:     Josh Poimboeuf <jpoimboe@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>
Subject: Re: [RFC PATCH v3] perf/x86: make perf callchain work without
 CONFIG_FRAME_POINTER
Message-ID: <20190419094313.GO7905@worktop.programming.kicks-ass.net>
References: <20190418160730.11901-1-kasong@redhat.com>
 <20190419005821.ttlpav7gviaed7am@treble>
 <CACPcB9fsYqs+xaj6w6ExKdpi-Pk4RqiD5HxTyURpee_arYQANg@mail.gmail.com>
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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@redhat.com> 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.

> > +       if (perf_hw_regs(regs)) {
> > +               if (perf_callchain_store(entry, regs->ip))
> > +                       return;
> > +               unwind_start(&state, current, regs, NULL);
> > +       } else {
> > +               unwind_start(&state, current, NULL, (void *)regs->sp);
> > +       }

> >  #define perf_arch_fetch_caller_regs(regs, __ip)                {       \
> >         (regs)->ip = (__ip);                                    \
> > +       (regs)->sp = (unsigned long)__builtin_frame_address(0); \
> >         (regs)->cs = __KERNEL_CS;                               \
> >         regs->flags = 0;                                        \
> >  }