linux-kernel - Re: [GIT PULL] x86/asm changes for v5.6

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Date:   Tue, 28 Jan 2020 21:14:15 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Tony Luck <tony.luck@...el.com>, Borislav Petkov <bp@...e.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] x86/asm changes for v5.6


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Tue, Jan 28, 2020 at 11:51 AM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> >         ALTERNATIVE_2 \
> >                 "cmp  $680, %rdx ; jb 3f ; cmpb %dil, %sil; je 4f", \
> >                 "movq %rdx, %rcx ; rep movsb; retq", X86_FEATURE_FSRM, \
> >                 "cmp $0x20, %rdx; jb 1f; movq %rdx, %rcx; rep movsb; retq", X86_FEATURE_ERMS
> 
> Note the UNTESTED part.
> 
> In particular, I didn't check what the priority for the alternatives
> is. Since FSRM being set always implies ERMS being set too, it may be
> that the ERMS case is always picked with the above code.
> 
> So maybe the FSRM and ERMS lines need to be switched around, and
> somebody should add a comment to the ALTERNATIVE_2 macro about the
> priority rules for feature1 vs feature2 when both are set..
> 
> IOW, testing most definitely required for that patch suggestion of mine..

Understood, thanks!

	Ingo