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

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Date:   Wed, 18 Aug 2021 14:04:41 -0700
From:   Thiago Macieira <thiago.macieira@...el.com>
To:     Borislav Petkov <bp@...en8.de>,
        "Bae, Chang Seok" <chang.seok.bae@...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 Wednesday, 18 August 2021 13:43:50 PDT Bae, Chang Seok wrote:
> > 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.

That assumes the kernel will always enable the bits in XCR0, like it is doing 
today and with your patch, because modifying it is a VM exit.

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.

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel DPG Cloud Engineering