Re: [PATCH 01/16] powerpc: Replace unreachable() with it's builtin variant in WARN_ON()

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

Christophe Leroy wrote:
> 
> 
> Le 18/08/2022 à 12:46, Naveen N. Rao a écrit :
>> Christophe Leroy wrote:
>>>
>>>
>>> Le 08/08/2022 à 13:48, Sathvika Vasireddy a écrit :
>>>> objtool is throwing *unannotated intra-function call*
>>>> warnings with a few instructions that are marked
>>>> unreachable. Replace unreachable() with __builtin_unreachable()
>>>> to fix these warnings, as the codegen remains same
>>>> with unreachable() and __builtin_unreachable().
>>>
>>> I think it is necessary to explain why using unreachable() is not 
>>> necessary for powerpc, or even why using unreachable() is wrong.
>>>
>>> Allthough we are getting rid of the problem here by replacing 
>>> unreachable() by __builtin_unreachable(), it might still be a problem 
>>> in core parts of kernel which still use unreachable.
>> 
>> I did a kernel build with this series applied, with a variant of 
>> ppc64le_defconfig. I then did another build with the same config, but 
>> with the below hunk to disable objtool:
>> 
>> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
>> index 6be2e68fa9eb64..4c466acdc70d4c 100644
>> --- a/arch/powerpc/Kconfig
>> +++ b/arch/powerpc/Kconfig
>> @@ -237,8 +237,6 @@ config PPC
>>         select HAVE_MOD_ARCH_SPECIFIC
>>         select HAVE_NMI                         if PERF_EVENTS || (PPC64 
>> && PPC_BOOK3S)
>>         select HAVE_OPTPROBES
>> -       select HAVE_OBJTOOL                     if PPC32 || MPROFILE_KERNEL
>> -       select HAVE_OBJTOOL_MCOUNT              if HAVE_OBJTOOL
>>         select HAVE_PERF_EVENTS
>>         select HAVE_PERF_EVENTS_NMI             if PPC64
>>         select HAVE_PERF_REGS
>> 
>> This has the effect of disabling annotations for unreachable().
>> 
>> When I compared the resulting object files, I did not see changes in 
>> codegen relating to the annotation, like we do with using unreachable() 
>> in __WARN_FLAGS().
>> 
>> More specifically, arch/powerpc/kvm/book3s.o:kvmppc_h_logical_ci_load() 
>> uses BUG(), and the generated code remains the same with/without the 
>> unreachable() annotation.
>> 
>> This suggests that the bad codegen we are seeing with the annotation in 
>> unreachable() is limited to its use in __WARN_FLAGS(), which I suspect 
>> is due to an interaction with the use of asm_volatile_goto() for 
>> WARN_ENTRY().
>> 
>> If I revert this patch (patch 01/16), gcc seems to add a label 8 bytes 
>> before _some_ function in this object file, which happens to hold a 
>> relocation against .TOC., and emits a bl to that symbol. Otherwise, gcc 
>> either emits no new instruction for the annotation, or a 'nop' in some 
>> cases.
>> 
>> If I add a 'nop' between WARN_ENTRY() and unreachable() in 
>> __WARN_FLAGS(), or convert WARN_ENTRY to BUG_ENTRY thereby removing use 
>> of asm_volatile_goto(), the problem goes away and no bl is emitted:
>> 
>> diff --git a/arch/powerpc/include/asm/bug.h 
>> b/arch/powerpc/include/asm/bug.h
>> index 61a4736355c244..88e0027c20ba5c 100644
>> --- a/arch/powerpc/include/asm/bug.h
>> +++ b/arch/powerpc/include/asm/bug.h
>> @@ -99,6 +99,7 @@
>>         __label__ __label_warn_on;                              \
>>                                                                 \
>>         WARN_ENTRY("twi 31, 0, 0", BUGFLAG_WARNING | (flags), 
>> __label_warn_on); \
>> +       __asm__ __volatile__("nop");                            \
>>         unreachable();                                          \
>>                                                                 \
>> __label_warn_on:
>> 
>> 
>> In summary, I think the annotation itself is fine and we are only seeing 
>> an issue with its usage after WARN_ENTRY() due to use of 
>> asm_volatile_goto. Other uses of unreachable() don't seem to exhibit 
>> this problem.
>> 
>> As such, I think this patch is appropriate for this series, though I 
>> think we should capture some of this information in the changelog.
>> 
>> Note also that if and when we start utlizing the annotation, if we 
>> classify twui as INSN_BUG, this change will continue to be appropriate.
>> 
> 
> INSN_TRAP instead of INSN_BUG ?

INSN_BUG, in line with your suggestion here:
http://lkml.kernel.org/r/ff623097-9f18-3914-5eae-bc6e4cd1510f@csgroup.eu

Peter was of the opinion that INSN_TRAP may not be what we want:
http://lkml.kernel.org/r/YsLSU6idNME/BtwH@hirez.programming.kicks-ass.net

If we classify twui as INSN_BUG, then objtool will know to stop control 
flow here without the need for an annotation. Parsing extable will 
then show that control flow continues with the label subsequently.


- Naveen