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-...@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
