linux-kernel - Re: [PATCH v3 1/2] x86/fpu: track AVX-512 usage of tasks

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Date:   Fri, 16 Nov 2018 15:10:13 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Li, Aubrey" <aubrey.li@...ux.intel.com>,
        Aubrey Li <aubrey.li@...el.com>, tglx@...utronix.de,
        mingo@...hat.com, peterz@...radead.org, hpa@...or.com
Cc:     ak@...ux.intel.com, tim.c.chen@...ux.intel.com,
        arjan@...ux.intel.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] x86/fpu: track AVX-512 usage of tasks

On 11/15/18 4:21 PM, Li, Aubrey wrote:
> "Core cycles where the core was running with power delivery for license
> level 2 (introduced in Skylake Server microarchitecture). This includes
> high current AVX 512-bit instructions."
> 
> I translated license level 2 to frequency drop.

BTW, the "high" in that text: "high-current AVX 512-bit instructions" is
talking about high-current, not "high ... instructions" or high-numbered
registers.  I think that might be the source of some of the confusion
about which XSAVE state needs to be examined.

Just to be clear: there are 3 AVX-512 XSAVE states:

        XFEATURE_OPMASK,
        XFEATURE_ZMM_Hi256,
        XFEATURE_Hi16_ZMM,

I honestly don't know what XFEATURE_OPMASK does.  It does not appear to
be affected by VZEROUPPER (although VZEROUPPER's SDM documentation isn't
looking too great).

But, XFEATURE_ZMM_Hi256 is used for the upper 256 bits of the
registers ZMM0-ZMM15.  Those are AVX-512-only registers.  The only way
to get data into XFEATURE_ZMM_Hi256 state is by using AVX512 instructions.

XFEATURE_Hi16_ZMM is the same.  The only way to get state in there is
with AVX512 instructions.

So, first of all, I think you *MUST* check XFEATURE_ZMM_Hi256 and
XFEATURE_Hi16_ZMM.  That's without question.

It's probably *possible* to run AVX512 instructions by loading state
into the YMM register and then executing AVX512 instructions that only
write to memory and never to register state.  That *might* allow
XFEATURE_Hi16_ZMM and XFEATURE_ZMM_Hi256 to stay in the init state, but
for the frequency to be affected since AVX512 instructions _are_
executing.  But, there's no way to detect this situation from XSAVE
states themselves.