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

Nathan Chancellor
2019-06-26T05:10:44Z

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@chromium.org/
>
> 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