linux-kernel - Linux patch disabling AVX when eagerfpu is turned off

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Message-ID: <02fd01d15437$233056c0$69910440$@ravellosystems.com>
Date:	Thu, 21 Jan 2016 12:33:13 +0200
From:	"Leonid Shatz" <leonid.shatz@...ellosystems.com>
To:	"'Ingo Molnar'" <mingo@...nel.org>, <linux-kernel@...r.kernel.org>,
	"'Yu-cheng Yu'" <yu-cheng.yu@...el.com>,
	"'Andy Lutomirski'" <luto@...capital.net>,
	"'Borislav Petkov'" <bp@...en8.de>,
	"'Dave Hansen'" <dave.hansen@...ux.intel.com>,
	"'Fenghua Yu'" <fenghua.yu@...el.com>,
	"'H. Peter Anvin'" <hpa@...or.com>,
	"'Linus Torvalds'" <torvalds@...ux-foundation.org>,
	"'Oleg Nesterov'" <oleg@...hat.com>,
	"'Peter Zijlstra'" <peterz@...radead.org>,
	"'Quentin Casasnovas'" <quentin.casasnovas@...cle.com>,
	"'Ravi V. Shankar'" <ravi.v.shankar@...el.com>,
	"'Sai Praneeth Prakhya'" <sai.praneeth.prakhya@...el.com>,
	"'Thomas Gleixner'" <tglx@...utronix.de>
Cc:	<liran.alon@...ellosystems.com>,
	<mendel.aizner@...ellosystems.com>,
	"'Elazar Leibovich'" <elazar.leibovich@...ellosystems.com>,
	"'Eyal Moscovici'" <eyal.moscovici@...ellosystems.com>,
	<gil.hoffer@...ellosystems.com>
Subject: Linux patch disabling AVX when eagerfpu is turned off - possibly not required

Regarding the patch disabling AVX when eagerfpu is off:

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=3
94db20ca240741a08d472173db13d6f6a6e5a28

We have a suggestion that Intel documentation related to usage of CR0.TS bit
may be not properly updated with additional support of AVX/AVX2/etc
instructions. In fact, there is certain ambiguity about the causes of #NM
exception in other sections of Intel documentation. Based on the chapter 2.4
INSTRUCTION EXCEPTION SPECIFICATION of Intel 64 and IA-32 Architectures
Software Developer's Manual, all SSE/SSE2/SSE4/MMX/AVX/etc. instructions,
which modify the FPU/MMX/AVX state are supposed to generate #NM exception
when CR0.TS = 1.

In addition, based on AMD documentation, the #NM exception is generated for
both SSE and AVX instructions in protected mode - refer to AMD64
Architecture Programmer's Manual Volume 4: 128-Bit and 256-Bit Media
Instructions and also "AMD64 Architecture
Programmer's Manual Volume 2: System Programming" Chapter 3.1 (Task Switched
(TS) Bit.3. When an attempt is made to execute an x87 or media instruction
while TS=1, a device-not-available exception (#NM) occurs).

Also, in our limited internal test on Intel I7 processor (repeated on two
different machines), while executing the AVX VMOVAPS instruction with CR0.TS
bit set, we've observed the #NM exception being always generated (although
we didn't perform all-inclusive tests for entire set of AVX and similar new
instructions).

In view of above findings we would like to suggest to double check if
disabling AVX together with "eagerfpu off" is actually required and is a
real necessity. It would be helpful to consult with Intel engineers
regarding related design details.

Sincerely,
Leonid Shatz,
Seniour Software Enigneer, Hypervisor Team,
Ravello Systems, Inc.