Re: [PATCH v6 1/3] x86/fpu: track AVX-512 usage of tasks

From: Dave Hansen
Date: Tue Dec 18 2018 - 12:14:08 EST

On 12/18/18 7:32 AM, Thomas Gleixner wrote:
> What exactly prevents a 32bit kernel from having the AVX512 feature bit
> set? And if it cannot be set on 32bit, then why are you compiling that code
> in at all?

There are three different AVX-512 states (and three bits) which Aubrey's
patch checks. All three have different rules. Here's a summary along
with some relevant SDM quotes from Vol1-13.6.

Opmask state: All opmask registers can be set in 32-bit mode.
ZMM_Hi256 state: "An execution of XRSTOR or XRSTORS outside 64-bit mode
does not update ZMM8_HâZMM15_H." This implies that
ZMM0_H-ZMM7_H *are* updated in 32-bit mode.
Hi16_ZMM state: "Outside 64-bit mode, Hi16_ZMM state is always in its
initial configuration."

All of Hi16_ZMM and *part* of ZMM_Hi256 can not be practically used in
32-bit mode. But, even using part of ZMM_Hi256 means the xfeature bit
will be set.

So, 2/3 of the features can be used in 32-bit mode. Nothing that I can
find _prevents_ those features from being used in 32-bit mode.

Aubrey, do you have information to the contrary?