linux-kernel - Re: [PATCH v2 0/2] x86/asm: avoid register pressure from static_cpu

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Message-ID: <YTsif1FeOEyFJnqc@zn.tnic>
Date:   Fri, 10 Sep 2021 11:16:47 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     "H. Peter Anvin (Intel)" <hpa@...or.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        x86@...nel.org
Subject: Re: [PATCH v2 0/2] x86/asm: avoid register pressure from
 static_cpu_has()

On Thu, Sep 09, 2021 at 03:08:16PM -0700, H. Peter Anvin (Intel) wrote:
> 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.
> 
> Explicitly force gcc to use immediate (rip-relative) addressing for
> the fallback path, thus removing any possible register use from
> static_cpu_has().

Right, maybe I'm missing something but what is wrong with the immediate
addressing variant, i.e., that thing:

	testb  $0x8,0xffffffff89346eea

and you need to *force* %rip-relative?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette