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

Peter Zijlstra
2019-06-26T09:55:49Z

On Wed, Jun 26, 2019 at 11:24:32AM +0200, Peter Zijlstra 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 :/

And I know nobody reads comments (I don't either), but I did write one
on this as it happens.

See include/linux/jump_label.h:399

/*
* Combine the right initial value (type) with the right branch order
* to generate the desired result.
*
*
* type\branch| likely (1) | unlikely (0)
* -----------+-----------------------+------------------
* | |
* true (1) | ... | ...
* | NOP | JMP L
* | <br-stmts> | 1: ...
* | L: ... |
* | |
* | | L: <br-stmts>
* | | jmp 1b
* | |
* -----------+-----------------------+------------------
* | |
* false (0) | ... | ...
* | JMP L | NOP
* | <br-stmts> | 1: ...
* | L: ... |
* | |
* | | L: <br-stmts>
* | | jmp 1b
* | |
* -----------+-----------------------+------------------
*
* The initial value is encoded in the LSB of static_key::entries,
* type: 0 = false, 1 = true.
*
* The branch type is encoded in the LSB of jump_entry::key,
* branch: 0 = unlikely, 1 = likely.
*
* This gives the following logic table:
*
* enabled type branch instuction
* -----------------------------+-----------
* 0 0 0 | NOP
* 0 0 1 | JMP
* 0 1 0 | NOP
* 0 1 1 | JMP
*
* 1 0 0 | JMP
* 1 0 1 | NOP
* 1 1 0 | JMP
* 1 1 1 | NOP
*
* Which gives the following functions:
*
* dynamic: instruction = enabled ^ branch
* static: instruction = type ^ branch
*
* See jump_label_type() / jump_label_init_type().
*/

The case here is false-unlikely, which *should* then result in the
branch statements going out-of-line.