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: <C3EBA85C-8708-4BAD-BB78-C975250BEFFF@intel.com>
Date:   Wed, 18 Aug 2021 21:12:06 +0000
From:   "Bae, Chang Seok" <chang.seok.bae@...el.com>
To:     "Macieira, Thiago" <thiago.macieira@...el.com>
CC:     Borislav Petkov <bp@...en8.de>,
        "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 14:04, Thiago Macieira <thiago.macieira@...el.com> wrote:
> But it's not the only possible solution. A future kernel could decide to leave 
> some bits off and only enable upon request. That's how macOS/Darwin does its 
> AVX512 support.

Even if XCR0 is ever switched, doesn’t XGETBV(0) return it for the *current*
task?

Thanks,
Chang