linux-kernel - Re: [PATCH v9 12/26] x86/fpu/xstate: Use feature disable (XFD) to protect dynamic user state

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Message-ID: <BCC327C2-CF9F-4910-B626-315E515E9A3A@intel.com>
Date:   Wed, 18 Aug 2021 20:43:50 +0000
From:   "Bae, Chang Seok" <chang.seok.bae@...el.com>
To:     Borislav Petkov <bp@...en8.de>,
        "Macieira, Thiago" <thiago.macieira@...el.com>
CC:     "Lutomirski, Andy" <luto@...nel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "Brown, Len" <len.brown@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v9 12/26] x86/fpu/xstate: Use feature disable (XFD) to
 protect dynamic user state

On Aug 18, 2021, at 10:46, Borislav Petkov <bp@...en8.de> wrote:
> On Wed, Aug 18, 2021 at 10:20:58AM -0700, Thiago Macieira wrote
>> The way the API to userspace is implemented, the only way to find
>> out if the kernel supports a given state is to enable it. It's not
>> currently possible to ask "do you support AMX tile data?"
> 
> Then our API needs improving. An app should be able to ask the kernel
> "Do you support AMX?" get a proper answer and act accordingly.

Maybe I’m missing something, but I wonder what’s the difference from reading
XCR0.

Thanks,
Chang