linux-kernel - Re: [PATCH v2 22/22] x86/fpu/xstate: Introduce boot-parameters for control some state component support

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Date:   Thu, 19 Nov 2020 21:02:26 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     "Chang S. Bae" <chang.seok.bae@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...e.de>,
        Andrew Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        Len Brown <len.brown@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v2 22/22] x86/fpu/xstate: Introduce boot-parameters for
 control some state component support

On Thu, Nov 19, 2020 at 3:37 PM Chang S. Bae <chang.seok.bae@...el.com> wrote:
>
> Rename XFEATURE_MASK_USER_SUPPORTED to XFEATURE_MASK_USER_ENABLED to
> literally align with new boot-parameters.
>
> "xstate.disable=0x60000" will disable AMX on a system that has AMX compiled
> into XFEATURE_MASK_USER_ENABLED.
>
> "xstate.enable=0x60000" will enable AMX on a system that does NOT have AMX
> compiled into XFEATURE_MASK_USER_ENABLED (assuming the kernel is new enough
> to support this feature).
>

What's the purpose of xstate.enable?  I can't really imagine it's
useful for AMX.  I suppose it could be useful for hypothetical
post-AMX features, but that sounds extremely dangerous.  Intel has
changed its strategy so many times on XSTATE extensibility that I find
it quite hard to believe that supporting unknown states is wise.