linux-kernel - Re: [PATCH 02/19] x86, fpu: Wrap get_xsave

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Message-ID: <20150529185134.GD27501@gmail.com>
Date:	Fri, 29 May 2015 20:51:34 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Andy Lutomirski <luto@...capital.net>, Dave Hansen <dave@...1.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Suresh Siddha <sbsiddha@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 02/19] x86, fpu: Wrap get_xsave_addr() to make it safer


* Borislav Petkov <bp@...en8.de> wrote:

> On Thu, May 28, 2015 at 06:05:33PM -0700, Andy Lutomirski wrote:
> > I would propose that we take the opposite approach and just ban
> > eagerfpu=off when MPX is enabled.  We could then take the next step
> > and default eagerfpu=on for everyone and, if nothing breaks, then just
> > delete lazy mode entirely.
> > 
> > I suspect we'd have to go back to Pentium 3 or earlier to find a CPU
> > on which lazy mode is actually a good idea.
> 
> Last time I checked (and ran some benchmarks) it was only a minute
> slowdown so I say we kill lazy mode if it means significant code
> complexity drop.
> 
> Can I also emulate Greg here and suggest that Pentium 3 people should
> buy newer hw? They should think about the environment, if nothing else.
> 
> :-P

I went back as far as Athon64 and the CR0 manipulation and CR0 faults are overly 
expensive there too.

Ok, you guys convinced me, I'll do a patch for this in tip:x86/fpu, and then 
people can benchmark it.

Thanks,

	Ingo
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