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SubjectRe: [PATCH] x86/asm: pessimize the pre-initialization case in static_cpu_has()
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On 9/9/21 10:01 AM, Borislav Petkov wrote:
> On Wed, Sep 08, 2021 at 10:17:16AM -0700, H. Peter Anvin (Intel) wrote:
>
>> Subject: Re: [PATCH] x86/asm: pessimize the pre-initialization case in static_cpu_has()
>
> "pessimize" huh? :)
>
> Why not simply
>
> "Do not waste registers in the pre-initialization case..."
>

Because it is shorter and thus can fit more contents

> ?
>
>> gcc will sometimes manifest the address of boot_cpu_data in a register
>> as part of constant propagation. When multiple static_cpu_has() are
>> used this may foul the mainline code with a register load which will
>> only be used on the fallback path, which is unused after
>> initialization.
>
> So a before-after thing looks like this here:
>
> before:
>
> ffffffff89696517 <.altinstr_aux>:
> ffffffff89696517: f6 05 cb 09 cb ff 80 testb $0x80,-0x34f635(%rip) # ffffffff89346ee9 <boot_cpu_data+0x69>
> ffffffff8969651e: 0f 85 fc 3e fb ff jne ffffffff8964a420 <intel_pmu_init+0x14e7>
> ffffffff89696524: e9 ee 3e fb ff jmp ffffffff8964a417 <intel_pmu_init+0x14de>
> ffffffff89696529: f6 45 6a 08 testb $0x8,0x6a(%rbp)
> ffffffff8969652d: 0f 85 45 b9 97 f7 jne ffffffff81011e78 <intel_pmu_lbr_filter+0x68>
> ffffffff89696533: e9 95 b9 97 f7 jmp ffffffff81011ecd <intel_pmu_lbr_filter+0xbd>
> ffffffff89696538: 41 f6 44 24 6a 08 testb $0x8,0x6a(%r12)
> ffffffff8969653e: 0f 85 d3 bc 97 f7 jne ffffffff81012217 <intel_pmu_store_lbr+0x77>
> ffffffff89696544: e9 d9 bc 97 f7 jmp ffffffff81012222 <intel_pmu_store_lbr+0x82>
> ffffffff89696549: 41 f6 44 24 6a 08 testb $0x8,0x6a(%r12)
>
> after:
>
> ffffffff89696517 <.altinstr_aux>:
> ffffffff89696517: f6 04 25 e9 6e 34 89 testb $0x80,0xffffffff89346ee9
> ffffffff8969651e: 80
> ffffffff8969651f: 0f 85 fb 3e fb ff jne ffffffff8964a420 <intel_pmu_init+0x14e7>
> ffffffff89696525: e9 ed 3e fb ff jmp ffffffff8964a417 <intel_pmu_init+0x14de>
> ffffffff8969652a: f6 04 25 ea 6e 34 89 testb $0x8,0xffffffff89346eea
> ffffffff89696531: 08
> ffffffff89696532: 0f 85 37 b9 97 f7 jne ffffffff81011e6f <intel_pmu_lbr_filter+0x5f>
> ffffffff89696538: e9 89 b9 97 f7 jmp ffffffff81011ec6 <intel_pmu_lbr_filter+0xb6>
> ffffffff8969653d: f6 04 25 ea 6e 34 89 testb $0x8,0xffffffff89346eea
> ffffffff89696544: 08
> ffffffff89696545: 0f 85 b5 bc 97 f7 jne ffffffff81012200 <intel_pmu_store_lbr+0x70>
> ffffffff8969654b: e9 bb bc 97 f7 jmp ffffffff8101220b <intel_pmu_store_lbr+0x7b>
> ffffffff89696550: f6 04 25 ea 6e 34 89 testb $0x8,0xffffffff89346eea
>
> so you're basically forcing an immediate thing.
>
> And you wanna get rid of the (%<reg>) relative addressing and force it
> to be rip-relative.
>
>> Explicitly force gcc to use immediate (rip-relative) addressing for
>
> Right, the rip-relative addressing doesn't happen here:
>

Indeed it doesn't (egg on my face), nor does it turn out is there
currently a way to do so (just adding (%%rip) breaks i386, and there is
no equivalent to %{pP} which adds the suffix). Let me fix both; will
have a patchset shortly.

-hpa

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