Messages in this thread Patch in this message | | | Date | Wed, 8 Feb 2023 22:21:12 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v3 3/4] x86/alternative: Rewrite optimize_nops() some |
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On Wed, Feb 08, 2023 at 10:08:12PM +0100, Peter Zijlstra wrote: > On Wed, Feb 08, 2023 at 09:44:04PM +0100, Peter Zijlstra wrote: > > > [ 11.584069] SMP alternatives: ffffffff82000095: [0:20) optimized NOPs: eb 12 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc > > [ 11.590068] SMP alternatives: ffffffff820001f3: [0:20) optimized NOPs: eb 12 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc > > [ 11.720069] SMP alternatives: ffffffff8200189f: [0:20) optimized NOPs: eb 12 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc > > [ 11.731069] SMP alternatives: ffffffff820019ae: [0:20) optimized NOPs: eb 12 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc > > [ 11.738069] SMP alternatives: ffffffff82001a4a: [0:20) optimized NOPs: eb 12 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc > > [ 11.746069] SMP alternatives: ffffffff82001b2d: [0:20) optimized NOPs: eb 12 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc > > [ 11.766069] SMP alternatives: ffffffff82001d14: [0:20) optimized NOPs: eb 12 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc > > [ 11.770069] SMP alternatives: ffffffff82001dd5: [0:20) optimized NOPs: eb 12 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc > > [ 11.779069] SMP alternatives: ffffffff82001f35: [0:20) optimized NOPs: eb 12 cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc > > UNTRAIN_RET -- specifically RESET_CALL_DEPTH
19: 48 c7 c0 80 00 00 00 mov $0x80,%rax 20: 48 c1 e0 38 shl $0x38,%rax 24: 65 48 89 04 25 00 00 00 00 mov %rax,%gs:0x0 29: R_X86_64_32S pcpu_hot+0x10
Is ofc an atrocity.
We can easily trim that by 5 bytes to:
0: b0 80 mov $0x80,%al 2: 48 c1 e0 38 shl $0x38,%rax 6: 65 48 89 04 25 00 00 00 00 mov %rax,%gs:0x0
Who cares about the top bytes, we're explicitly shifting them out anyway. But that's still 15 bytes or so.
If it weren't for those pesky prefix penalties that would make exactly one instruction :-)
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h index e04313e89f4f..be792f9407b5 100644 --- a/arch/x86/include/asm/nospec-branch.h +++ b/arch/x86/include/asm/nospec-branch.h @@ -84,7 +84,7 @@ movq $-1, PER_CPU_VAR(pcpu_hot + X86_call_depth); #define RESET_CALL_DEPTH \ - mov $0x80, %rax; \ + movb $0x80, %al; \ shl $56, %rax; \ movq %rax, PER_CPU_VAR(pcpu_hot + X86_call_depth);
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