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

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Message-ID: <CALCETrVxi8nWZYryr-WcMiFiOH0x=4-BMfqEmgmHsr2kUw8yOQ@mail.gmail.com>
Date:	Thu, 21 Jan 2016 13:03:03 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...en8.de>,
	Eyal Moscovici <eyal.moscovici@...ellosystems.com>,
	"Ravi V. Shankar" <ravi.v.shankar@...el.com>,
	liran.alon@...ellosystems.com,
	Quentin Casasnovas <quentin.casasnovas@...cle.com>,
	Ingo Molnar <mingo@...nel.org>,
	Leonid Shatz <leonid.shatz@...ellosystems.com>,
	Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Fenghua Yu <fenghua.yu@...el.com>,
	gil.hoffer@...ellosystems.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Elazar Leibovich <elazar.leibovich@...ellosystems.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Dave Hansen <dave.hansen@...el.com>,
	mendel.aizner@...ellosystems.com,
	Peter Zijlstra <peterz@...radead.org>,
	Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: Linux patch disabling AVX when eagerfpu is turned off - possibly
 not required

On Jan 21, 2016 12:12 PM, "H. Peter Anvin" <hpa@...or.com> wrote:
>
> On 01/21/16 12:08, Dave Hansen wrote:
> > On 01/21/2016 02:33 AM, Leonid Shatz wrote:
> >> 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.
> >
> > Hi Leonid,
> >
> > Thanks for the report!
> >
> > Are you aware of any actual eagerfpu=off use in practice, or is this
> > mostly a theoretical concern?
> >
>
> Looking into this, FWIW.

I still think that we should default eagerfpu=on on all CPUs for one
release and then, when nothing breaks, delete all the old lazy code.
I'll respin and send that patch.

--Andy