lkml.org 
[lkml]   [2019]   [Jun]   [26]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    From
    Date
    SubjectRe: [PATCH] perf/x86/intel: Mark expected switch fall-throughs
    On Wed, Jun 26, 2019 at 2:24 AM Peter Zijlstra <peterz@infradead.org> wrote:
    >
    > On Tue, Jun 25, 2019 at 11:47:06PM +0200, Thomas Gleixner wrote:
    > > > On Tue, Jun 25, 2019 at 09:53:09PM +0200, Thomas Gleixner wrote:
    >
    > > > > 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
    >
    > > 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.
    >
    > That's pretty atrocious code-gen :/ Does LLVM support things like label
    > attributes? Back when we did jump labels GCC didn't, or rather, it
    > ignored it completely when combined with asm goto (and it might still).
    >
    > That is, would something like this:
    >
    > diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h
    > index 06c3cc22a058..1761b1e76ddc 100644
    > --- a/arch/x86/include/asm/jump_label.h
    > +++ b/arch/x86/include/asm/jump_label.h
    > @@ -32,7 +32,7 @@ static __always_inline bool arch_static_branch(struct static_key *key, bool bran
    > : : "i" (key), "i" (branch) : : l_yes);
    >
    > return false;
    > -l_yes:
    > +l_yes: __attribute__((cold));
    > return true;
    > }
    >
    > @@ -49,7 +49,7 @@ static __always_inline bool arch_static_branch_jump(struct static_key *key, bool
    > : : "i" (key), "i" (branch) : : l_yes);
    >
    > return false;
    > -l_yes:
    > +l_yes: __attribute__((hot));
    > return true;
    > }
    >
    > Help LLVM?

    So Clang definitely complains about putting attribute hot/cold on
    labels: https://godbolt.org/z/N-Z33Q
    In my test case I wasn't able to influence code gen with them though
    in GCC at -O2 or -O0. Maybe GCC has a test case that shows how they
    should work?
    --
    Thanks,
    ~Nick Desaulniers

    \
     
     \ /
      Last update: 2019-06-27 00:16    [W:2.825 / U:0.100 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site