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

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Date:   Tue, 18 Dec 2018 09:14:02 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        "Li, Aubrey" <aubrey.li@...ux.intel.com>
Cc:     Aubrey Li <aubrey.li@...el.com>, mingo@...hat.com,
        peterz@...radead.org, hpa@...or.com, ak@...ux.intel.com,
        tim.c.chen@...ux.intel.com, arjan@...ux.intel.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 1/3] x86/fpu: track AVX-512 usage of tasks

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_H–ZMM15_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?