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

[Ravi Bangoria <ravi.bangoria@amd.com>]

>>> 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.

This should resolve the DWARF unwinding issue with IBS PMUs:

--- a/arch/x86/events/amd/ibs.c
+++ b/arch/x86/events/amd/ibs.c
@@ -1523,8 +1523,11 @@ static int perf_ibs_handle_irq(struct perf_ibs *perf_ibs, struct pt_regs *iregs)
 			goto out;
 		}
 
-		set_linear_ip(&regs, ibs_data.regs[1]);
-		regs.flags |= PERF_EFLAGS_EXACT;
+		if (event->attr.sample_type & PERF_SAMPLE_IP) {
+			data.ip = ibs_data.regs[1];
+			data.sample_flags |= PERF_SAMPLE_IP;
+			regs.flags |= PERF_EFLAGS_EXACT;
+		}
 	}
 
 	if (((ibs_caps & IBS_CAPS_BIT63_FILTER) ||
---

However, there's no way to address the stack being out of sync with the
IBS RIP, since the IBS HW does not capture GPRs alongside the sample.

Thanks,
Ravi