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From:   Len Brown <lenb@kernel.org>
Date:   Mon, 24 May 2021 14:24:54 -0400
Message-ID: <CAJvTdKmOLb0pt2SDgAKfwjxtMzT=t=UoREoPRUmi=4MWzeJ+QA@mail.gmail.com>
Subject: Re: [PATCH v5 28/28] x86/fpu/amx: Clear the AMX state when appropriate
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     Andy Lutomirski <luto@kernel.org>,
        "Chang S. Bae" <chang.seok.bae@intel.com>,
        Borislav Petkov <bp@suse.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, X86 ML <x86@kernel.org>,
        "Brown, Len" <len.brown@intel.com>,
        "Liu, Jing2" <jing2.liu@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "Chen, Tim C" <tim.c.chen@intel.com>
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On Mon, May 24, 2021 at 1:39 PM Dave Hansen <dave.hansen@intel.com> wrote:

> >> might be considered nasty.

> > I'm not excited about burdening the generic idle path with a CPU
> > feature specific check that would need to be checked on every idle
> > entry.
>
> Me neither.
>
> But, the check itself should be cheap.  A cpu_feature_enabled(AMX) check
> will eliminate even the cost of a branch on systems without AMX.  You
> could probably even get fancy and also use a static branch that doesn't
> get enabled until the first AMX user shows up.

It isn't just the hardware run-time cost.
It is the source code complexity.
That code is 100% generic.

Len Brown, Intel Open Source Technology Center