linux-kernel - Re: [PATCH RFC 01/14] x86/asm: add iosubmit_cmds512() based on movdir64b CPU instruction

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Message-ID: <20191121103732.GA6540@zn.tnic>
Date:   Thu, 21 Nov 2019 11:37:32 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "Luck, Tony" <tony.luck@...el.com>,
        Dave Jiang <dave.jiang@...el.com>, dmaengine@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Vinod Koul <vkoul@...nel.org>, Jing Lin <jing.lin@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>,
        "Dey, Megha" <megha.dey@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>, Jens Axboe <axboe@...nel.dk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH RFC 01/14] x86/asm: add iosubmit_cmds512() based on
 movdir64b CPU instruction

On Wed, Nov 20, 2019 at 05:32:51PM -0800, Dan Williams wrote:
> True, but that would be a driver coding mistake flagged by the
> WARN_ON_ONCE, and the failure is static. The driver must check for
> static_cpu_has(X86_FEATURE_MOVDIR64B) once at init,

So if you do that at driver init time, you don't need the static variant
- simply use boot_cpu_has(). But if this function is going to be used on
other platforms, then you need the check in the function and it must be
static_cpu_has() for speed.

The static_cpu_has() thing is a soft-of once check anyway because it
gets patched by alternatives and after that it is 0 overhead.

-- 
Regards/Gruss,
    Boris.

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