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

Andy Lutomirski
2021-08-19T01:21:39Z

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
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