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: <ea7eddc4-decd-47b7-b98b-c04c441495d9@www.fastmail.com>
Date:   Wed, 18 Aug 2021 18:21:11 -0700
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Thiago Macieira" <thiago.macieira@...el.com>,
        "Borislav Petkov" <bp@...en8.de>,
        "Bae, Chang Seok" <chang.seok.bae@...el.com>
Cc:     "Thomas Gleixner" <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...nel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "Brown, Len" <len.brown@...el.com>,
        "Dave Hansen" <dave.hansen@...el.com>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v9 12/26] x86/fpu/xstate: Use feature disable (XFD) to protect dynamic user state



On Wed, Aug 18, 2021, at 2:04 PM, Thiago Macieira wrote:
> 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.

The fact that Darwin does this strongly suggests that real programs can handle it, which increases my inclination for Linux to do the same thing.

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