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

[Thiago Macieira]

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