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

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Message-ID: <CAJvTdKk-d2rpsAYiPg7iJNZ=sEyhjBnGrT3Hy8Mt5G1TkEJRDw@mail.gmail.com>
Date:   Fri, 26 Mar 2021 11:27:20 -0400
From:   Len Brown <lenb@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Borislav Petkov <bp@...e.de>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>, X86 ML <x86@...nel.org>,
        "Brown, Len" <len.brown@...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 7:10 PM Dave Hansen <dave.hansen@...el.com> wrote:
>
> On 3/25/21 3:59 PM, Len Brown wrote:
> > We call AMX a "simple state feature" -- it actually requires NO KERNEL ENABLING
> > above the generic state save/restore to fully support userspace AMX
> > applications.
> >
> > While not all ISA extensions can be simple state features, we do expect
> > future features to share this trait, and so we want to be sure that it is simple
> > to update the kernel to turn those features on (and when necessary, off).
>
> From some IRC chats with Thomaas and Andy, I think it's safe to say that
> they're not comfortable blindly enabling even our "simple features".  I
> think we're going to need at least some additional architecture to get
> us to a point where everyone will be comfortable.

Hi Dave,

There is no code in this patch series, including patch 22, that enables
an unvalidated feature by default.

Yes, I fully accept that patch 22 allows a user to enable something
that a distro didn't validate.

If there is a new requirement that the kernel cmdline not allow anything
that a distro didn't explicitly validate, then about 99.9% of the kernel cmdline
options that exist today would need to be removed.

Does such a requirement exist, or does it not?

thanks,
-Len