Re: [PATCH] perf/x86/intel: Mark expected switch fall-throughs

[Nathan Chancellor]

On Tue, Jun 25, 2019 at 11:47:06PM +0200, Thomas Gleixner wrote:
> On Tue, 25 Jun 2019, Nathan Chancellor wrote:
> > On Tue, Jun 25, 2019 at 09:53:09PM +0200, Thomas Gleixner wrote:
> > > 
> > > But can the script please check for a minimal clang version required to
> > > build that thing.
> > > 
> > > The default clang-3.8 which is installed on Debian stretch explodes. The
> > > 6.0 variant from backports works as advertised.
> > > 
> > 
> > Hmmm interesting, I test a lot of different distros using Docker
> > containers to make sure the script works universally and that includes
> > Debian stretch, which is the stress tester because all of the packages
> > are older. I install the following packages then run the following
> > command and it works fine for me (just tested):
> > 
> > $ apt update && apt install -y --no-install-recommends ca-certificates \
> > ccache clang cmake curl file gcc g++ git make ninja-build python3 \
> > texinfo zlib1g-dev
> > $ ./build-llvm.py
> > 
> > If you could give me a build log, I'd be happy to look into it and see
> > what I can do.
> 
> I can produce one tomorrow.
>  

Great, thank you!

> > > Kernel builds with the new shiny compiler. Jump labels seem to be enabled.
> > > 
> > > It complains about a few type conversions:
> > > 
> > >  arch/x86/kvm/mmu.c:4596:39: warning: implicit conversion from 'int' to 'u8' (aka 'unsigned char') changes value from -205 to 51 [-Wconstant-conversion]
> > >                 u8 wf = (pfec & PFERR_WRITE_MASK) ? ~w : 0;
> > >                    ~~                               ^~
> > > 
> > 
> > Yes, there was a patch sent to try and fix this but it was rejected by
> > the maintainers:
> > 
> > https://github.com/ClangBuiltLinux/linux/issues/95
> > 
> > https://lore.kernel.org/lkml/20180619192504.180479-1-mka@xxxxxxxxxxxx/
> 
> Just looked through it. I don't think it's an outright reject. Paolo was
> not totally against it and then the whole discussion degraded into bikeshed
> painting and bitching about compiler error messaged. Try again or should I?
> 

Might be worth having you chime in, given that is the only instance of
that type of warning that I see in my set of builds (I fixed the rest:
https://github.com/ClangBuiltLinux/linux/issues?q=label%3A-Wconstant-conversion)

> > > but it also makes objtool unhappy:
> > > 
> > >  arch/x86/events/intel/core.o: warning: objtool: intel_pmu_nhm_workaround()+0xb3: unreachable
> instruction
> > >  kernel/fork.o: warning: objtool: free_thread_stack()+0x126: unreachable instruction
> > >  mm/workingset.o: warning: objtool: count_shadow_nodes()+0x11f: unreachable instruction
> > >  arch/x86/kernel/cpu/mtrr/generic.o: warning: objtool: get_fixed_ranges()+0x9b: unreachable
> instruction
> > >  arch/x86/kernel/platform-quirks.o: warning: objtool: x86_early_init_platform_quirks()+0x84:
> unreachable instruction
> > >  drivers/iommu/irq_remapping.o: warning: objtool: irq_remap_enable_fault_handling()+0x1d:
> unreachable instruction
> 
> > Unfortunately, we have quite a few of those outstanding, it's probably
> > time to start really taking a look at them:
> > 
> > https://github.com/ClangBuiltLinux/linux/labels/objtool
> 
> I just checked two of them in the disassembly. In both cases it's jump
> label related. Here is one:
> 
>       asm volatile("1: rdmsr\n"
>  410:   b9 59 02 00 00          mov    $0x259,%ecx
>  415:   0f 32                   rdmsr
>  417:   49 89 c6                mov    %rax,%r14
>  41a:   48 89 d3                mov    %rdx,%rbx
>       return EAX_EDX_VAL(val, low, high);
>  41d:   48 c1 e3 20             shl    $0x20,%rbx
>  421:   48 09 c3                or     %rax,%rbx
>  424:   0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
>  429:   eb 0f                   jmp    43a <get_fixed_ranges+0xaa>
>       do_trace_read_msr(msr, val, 0);
>  42b:   bf 59 02 00 00          mov    $0x259,%edi   <------- "unreachable"
>  430:   48 89 de                mov    %rbx,%rsi
>  433:   31 d2                   xor    %edx,%edx
>  435:   e8 00 00 00 00          callq  43a <get_fixed_ranges+0xaa>
>  43a:   44 89 35 00 00 00 00    mov    %r14d,0x0(%rip)        # 441 <get_fixed_ranges+0xb1>
> 
> Interestingly enough there are some more hunks of the same pattern in that
> function which look all the same. Those are not upsetting objtool. Josh
> might give an hint where to stare at.
> 
> Just for the fun of it I looked at the GCC output of the same file. It
> takes a different apporach:
> 
>       asm volatile("1: rdmsr\n"
>  c70:   b9 59 02 00 00          mov    $0x259,%ecx
>  c75:   0f 32                   rdmsr
>       return EAX_EDX_VAL(val, low, high);
>  c77:   48 c1 e2 20             shl    $0x20,%rdx
>  c7b:   48 89 d3                mov    %rdx,%rbx
>  c7e:   48 09 c3                or     %rax,%rbx
>  c81:   0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
>  c86:   48 89 1d 00 00 00 00    mov    %rbx,0x0(%rip)        # c8d <get_fixed_ranges.constprop.5+0x7d>
> 
> and the tracing code is completely out of line:
> 
>       do_trace_read_msr(msr, val, 0);
>  ce2:   31 d2                   xor    %edx,%edx
>  ce4:   48 89 de                mov    %rbx,%rsi
>  ce7:   bf 59 02 00 00          mov    $0x259,%edi
>  cec:   e8 00 00 00 00          callq  cf1 <get_fixed_ranges.constprop.5+0xe1>
>  cf1:   eb 93                   jmp    c86 <get_fixed_ranges.constprop.5+0x76>
> 
> which makes a lot of sense as the normal path (tracepoint disabled) just
> runs through linearly while in the clang version it has to jump around the
> tracepoint code.
> 
> The jump itself is not a problem, but what matters is the $I cache
> footprint. The GCC version hotpath fits in 3 cache lines while the Clang
> version unconditionally eats 4.2 of them. That's a huge difference.
> 
> > Thanks for trying it out and letting us know. Please keep us in the loop
> > if you happen to find anything amiss.
> 
> Will do.
> 
> Thanks,
> 
> 	tglx