Re: [DISCUSSION] Three problems behind broken "perf --call-graph dwarf" on AMD: IP and stack dump mismatch, libdw fails on lld's layout, and the unwinder fallback never runs

[Namhyung Kim <namhyung@kernel.org>]

On Thu, Aug 27, 2026 at 02:55:15PM -0700, Namhyung Kim wrote:
> Hello,
> 
> Thanks a lot for your detailed report!  It's good but let me jump into
> the action items directly.
> 
> On Fri, Aug 21, 2026 at 10:49:59PM +0100, Gennady Kupava wrote:
[SNIP]
> > 2. perf: precise events and DWARF call graphs are mutually exclusive on this
> >    hardware, so arguably perf should simply not let the two be combined.  I
> >    would suggest two rules rather than one, because a blanket refusal would
> >    make the common case worse:
> > 
> >    - for the default event, drop the P when --call-graph dwarf is requested,
> >      silently and on PMUs where precision means IBS.  Otherwise a plain
> >      "perf record -g --call-graph dwarf" starts failing outright on every AMD
> >      box, which is worse than today.  The s390 case in evlist.c suggests this
> >      kind of substitution is considered acceptable;
> > 
> >    - if the user asked for a precise event explicitly and also asked for
> >      DWARF call chains, refuse with a message that says why and what to do,
> >      instead of quietly producing a useless result.
> > 
> >    Note that this should be scoped to IBS.  On Intel, PEBS records the whole
> >    register set, so precise events and DWARF unwinding work together there
> >    and nothing needs restricting.  Frame-pointer call graphs are also fine
> >    with precise events - only the leaf frame is off.
> 
> I think we can enable both precise IP and dwarf callchains by using
> PERF_SAMPLE_REGS_INTR.  For unwinding, it should use PERF_REG_X86_IP
> from the REGS_INTR instead of sample.ip

Ok, it seems we already do this in the perf tools but it looks like the
kernel already overwrote the PERF_REG_X86_IP with the precise IP.  I
feel like we should fix the kernel.

Thanks,
Namhyung