linux-kernel - Re: x86 copy performance regression

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Message-ID: <CANn89i+u8jvfSQAQ=_JY0be56deJNhKgDWbqpDAvfm-i34qX9A@mail.gmail.com>
Date:   Fri, 26 May 2023 19:25:23 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>
Subject: Re: x86 copy performance regression

On Fri, May 26, 2023 at 7:17 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Fri, May 26, 2023 at 10:00 AM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > Let me go look at it some more. I *really* didn't want to make the
> > code worse for ERMS
>
> Oh well. I'll think about it some more in the hope that I can come up
> with something clever that doesn't make objtool hate me, but in the
> meantime let me just give you the "not clever" patch.
>
> It generates an annoying six-byte jump when the small 2-byte one would
> work just fine, but I guess only my pride is wounded.

arch/x86/lib/copy_user_64.S:34:2: error: invalid instruction mnemonic
'alternative'
 alternative "jae .Lunrolled", "jae .Llarge", ( 9*32+ 9)
 ^~~~~~~~~~~

I changed alternative to ALTERNATIVE to let it build.

 SYM_FUNC_START(rep_movs_alternative)
        cmpq $64,%rcx
-       jae .Lunrolled
+       ALTERNATIVE "jae .Lunrolled", "jae .Llarge", X86_FEATURE_ERMS

I will report test result soon, thanks !