Re: [PATCH v2] powerpc/64: BE option to use ELFv2 ABI for big endian kernels

["Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>]

Nicholas Piggin wrote:
> Excerpts from Michal Suchánek's message of May 4, 2021 6:17 am:
>> On Mon, May 03, 2021 at 11:34:25AM +0200, Michal Suchánek wrote:
>>> On Mon, May 03, 2021 at 09:11:16AM +0200, Michal Suchánek wrote:
>>> > On Mon, May 03, 2021 at 10:58:33AM +1000, Nicholas Piggin wrote:
>>> > > Excerpts from Michal Suchánek's message of May 3, 2021 2:57 am:
>>> > > > On Tue, Apr 28, 2020 at 09:25:17PM +1000, Nicholas Piggin wrote:
>>> > > >> Provide an option to use ELFv2 ABI for big endian builds. This works on
>>> > > >> GCC and clang (since 2014). It is less well tested and supported by the
>>> > > >> GNU toolchain, but it can give some useful advantages of the ELFv2 ABI
>>> > > >> for BE (e.g., less stack usage). Some distros even build BE ELFv2
>>> > > >> userspace.
>>> > > > 
>>> > > > Fixes BTFID failure on BE for me and the ELF ABIv2 kernel boots.
>>> > > 
>>> > > What's the BTFID failure? Anything we can do to fix it on the v1 ABI or 
>>> > > at least make it depend on BUILD_ELF_V2?
>>> > 
>>> > Looks like symbols are prefixed with a dot in ABIv1 and BTFID tool is
>>> > not aware of that. It can be disabled on ABIv1 easily.

Yes, I think BTF is generated by pahole, so we will need to add support 
for recognising dot symbols there.

>>> > 
>>> > Thanks
>>> > 
>>> > Michal
>>> > 
>>> > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
>>> > index 678c13967580..e703c26e9b80 100644
>>> > --- a/lib/Kconfig.debug
>>> > +++ b/lib/Kconfig.debug
>>> > @@ -305,6 +305,7 @@ config DEBUG_INFO_BTF
>>> >  	bool "Generate BTF typeinfo"
>>> >  	depends on !DEBUG_INFO_SPLIT && !DEBUG_INFO_REDUCED
>>> >  	depends on !GCC_PLUGIN_RANDSTRUCT || COMPILE_TEST
>>> > +	depends on !PPC64 || BUILD_ELF_V2
>>> >  	help
>>> >  	  Generate deduplicated BTF type information from DWARF debug info.
>>> >  	  Turning this on expects presence of pahole tool, which will convert
>>> > 
>>> > > 
>>> > > > 
>>> > > > Tested-by: Michal Suchánek <msuchanek@suse.de>
>>> > > > 
>>> > > > Also can we enable mprofile on BE now?
>>> > > > 
>>> > > > I don't see anything endian-specific in the mprofile code at a glance
>>> > > > but don't have any idea how to test it.
>>> > > 
>>> > > AFAIK it's just a different ABI for the _mcount call so just running
>>> > > some ftrace and ftrace with call graph should test it reasonably well.
>>> 
>>> It does not crash and burn but there are some regressions from LE to BE
>>> on the ftrace kernel selftest:
>>> 
>>> --- ftraceLE.txt	2021-05-03 11:19:14.830000000 +0200
>>> +++ ftraceBE.txt	2021-05-03 11:27:24.770000000 +0200
>>> @@ -7,8 +7,8 @@
>>>  [n] Change the ringbuffer size	[PASS]
>>>  [n] Snapshot and tracing setting	[PASS]
>>>  [n] trace_pipe and trace_marker	[PASS]
>>> -[n] Test ftrace direct functions against tracers	[UNRESOLVED]
>>> -[n] Test ftrace direct functions against kprobes	[UNRESOLVED]
>>> +[n] Test ftrace direct functions against tracers	[FAIL]
>>> +[n] Test ftrace direct functions against kprobes	[FAIL]

Strange to see those fail. I don't think my patch to enable ftrace 
direct functions is upstream yet ;)

>>>  [n] Generic dynamic event - add/remove kprobe events	[PASS]
>>>  [n] Generic dynamic event - add/remove synthetic events	[PASS]
>>>  [n] Generic dynamic event - selective clear (compatibility)	[PASS]
>>> @@ -16,10 +16,10 @@
>>>  [n] event tracing - enable/disable with event level files	[PASS]
>>>  [n] event tracing - restricts events based on pid notrace filtering	[PASS]
>>>  [n] event tracing - restricts events based on pid	[PASS]
>>> -[n] event tracing - enable/disable with subsystem level files	[PASS]
>>> +[n] event tracing - enable/disable with subsystem level files	[FAIL]
>>>  [n] event tracing - enable/disable with top level files	[PASS]
>>> -[n] Test trace_printk from module	[UNRESOLVED]
>>> -[n] ftrace - function graph filters with stack tracer	[PASS]
>>> +[n] Test trace_printk from module	[FAIL]
>>> +[n] ftrace - function graph filters with stack tracer	[FAIL]
>>>  [n] ftrace - function graph filters	[PASS]
>>>  [n] ftrace - function trace with cpumask	[PASS]
>>>  [n] ftrace - test for function event triggers	[PASS]
>>> @@ -27,7 +27,7 @@
>>>  [n] ftrace - function pid notrace filters	[PASS]
>>>  [n] ftrace - function pid filters	[PASS]
>>>  [n] ftrace - stacktrace filter command	[PASS]
>>> -[n] ftrace - function trace on module	[UNRESOLVED]
>>> +[n] ftrace - function trace on module	[FAIL]
>>>  [n] ftrace - function profiler with function tracing	[PASS]
>>>  [n] ftrace - function profiling	[PASS]
>>>  [n] ftrace - test reading of set_ftrace_filter	[PASS]
>>> @@ -44,10 +44,10 @@
>>>  [n] Kprobe event argument syntax	[PASS]
>>>  [n] Kprobe dynamic event with arguments	[PASS]
>>>  [n] Kprobes event arguments with types	[PASS]
>>> -[n] Kprobe event user-memory access	[UNSUPPORTED]
>>> +[n] Kprobe event user-memory access	[FAIL]
>>>  [n] Kprobe event auto/manual naming	[PASS]
>>>  [n] Kprobe dynamic event with function tracer	[PASS]
>>> -[n] Kprobe dynamic event - probing module	[UNRESOLVED]
>>> +[n] Kprobe dynamic event - probing module	[FAIL]
>>>  [n] Create/delete multiprobe on kprobe event	[PASS]
>>>  [n] Kprobe event parser error log check	[PASS]
>>>  [n] Kretprobe dynamic event with arguments	[PASS]
>>> @@ -57,11 +57,11 @@
>>>  [n] Kprobe events - probe points	[PASS]
>>>  [n] Kprobe dynamic event - adding and removing	[PASS]
>>>  [n] Uprobe event parser error log check	[PASS]
>>> -[n] test for the preemptirqsoff tracer	[UNSUPPORTED]
>>> -[n] Meta-selftest: Checkbashisms	[UNRESOLVED]
>>> +[n] test for the preemptirqsoff tracer	[FAIL]
>>> +[n] Meta-selftest: Checkbashisms	[FAIL]
>>>  [n] Test wakeup RT tracer	[PASS]
>>>  [n] Test wakeup tracer	[PASS]
>>> -[n] event trigger - test inter-event histogram trigger expected fail actions	[XFAIL]
>>> +[n] event trigger - test inter-event histogram trigger expected fail actions	[FAIL]
>>>  [n] event trigger - test field variable support	[PASS]
>>>  [n] event trigger - test inter-event combined histogram trigger	[PASS]
>>>  [n] event trigger - test multiple actions on hist trigger	[PASS]
>>> @@ -96,7 +96,8 @@
>>>  [n] (instance)  event tracing - enable/disable with event level files	[PASS]
>>>  [n] (instance)  event tracing - restricts events based on pid notrace filtering	[PASS]
>>>  [n] (instance)  event tracing - restricts events based on pid	[PASS]
>>> -[n] (instance)  event tracing - enable/disable with subsystem level files	[PASS]
>>> +[n] (instance)  event tracing - enable/disable with subsystem level files	[FAIL]
>>> +rmdir: failed to remove '/sys/kernel/tracing/instances/ftracetest.mceByV': Device or resource busy
>>>  [n] (instance)  ftrace - test for function event triggers	[PASS]
>>>  [n] (instance)  ftrace - function pid notrace filters	[PASS]
>>>  [n] (instance)  ftrace - function pid filters	[PASS]
>>> 
>>> I needed to add a test timeout to get this far because
>>> "event tracing - enable/disable with subsystem level files" gets stuck.

Does that test pass on LE for you? In the past, I have seen some lockups 
but will have to go revisit the tests...

>> 
>> There is some regression from mprofile on BE ABIv2
>> 
>> --- ftraceBEv2.txt	2021-05-03 18:11:01.100000000 +0200
>> +++ ftraceBE.mprofile.txt	2021-05-03 11:27:24.770000000 +0200
>> @@ -19,7 +19,7 @@
>>  [n] event tracing - enable/disable with subsystem level files	[FAIL]
>>  [n] event tracing - enable/disable with top level files	[PASS]
>>  [n] Test trace_printk from module	[FAIL]
>> -[n] ftrace - function graph filters with stack tracer	[PASS]
>> +[n] ftrace - function graph filters with stack tracer	[FAIL]
>>  [n] ftrace - function graph filters	[PASS]
>>  [n] ftrace - function trace with cpumask	[PASS]
>>  [n] ftrace - test for function event triggers	[PASS]
>> @@ -31,7 +31,7 @@
>>  [n] ftrace - function profiler with function tracing	[PASS]
>>  [n] ftrace - function profiling	[PASS]
>>  [n] ftrace - test reading of set_ftrace_filter	[PASS]
>> -[n] ftrace - Max stack tracer	[PASS]
>> +[n] ftrace - Max stack tracer	[FAIL]
>>  [n] ftrace - test for function traceon/off triggers	[PASS]
>>  [n] ftrace - test tracing error log support	[PASS]
>>  [n] Test creation and deletion of trace instances while setting an event	[PASS]
> 
> Hmm, I don't see an obvious case where the stack layout is based on
> BE when it should be using ELF ABI version, or ftrace_64_mprofile.S
> depends on LE. +Naveen

Yes, I don't think we ever enabled the stack tracer to work properly on 
LE and -mprofile-kernel. I will take a look tomorrow.

- Naveen