linux-kernel - Re: [PATCH v4 22/22] x86/fpu/xstate: Introduce boot-parameters to control state component support

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Message-ID: <CAJvTdK=OGALDso0H+asjgkjD_VaPNZzm+LpV+msM_i5aVUm_qw@mail.gmail.com>
Date:   Fri, 26 Mar 2021 11:33:47 -0400
From:   Len Brown <lenb@...nel.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Borislav Petkov <bp@...e.de>, Ingo Molnar <mingo@...nel.org>,
        X86 ML <x86@...nel.org>, "Brown, Len" <len.brown@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Documentation List <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v4 22/22] x86/fpu/xstate: Introduce boot-parameters to
 control state component support

On Thu, Mar 25, 2021 at 9:42 PM Andy Lutomirski <luto@...nel.org> wrote:

> Regardless of what you call AMX, AMX requires kernel enabling.

I submit, that after the generic XFD support is in place,
there is exactly 1 bit that needs to be flipped to enable
user applications to benefit from AMX.

I submit the patch that knows about AMX and double checks the
state size is superfluous.

I submit that updating /proc/cpuinfo is superfluous.

What AMX-specific kernel enabling did I miss?

> Specifically, it appears that leaving AMX in use in the XINUSE sense
> degrades system performance and/or power.

Please share the specifics about what performance or power issue you anticipate.

thanks,
-Len