linux-kernel - Re: [PATCH] x86/FPU: Fix FPU handling on legacy FPU machines

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Message-ID: <20160312150809.GA7015@gmail.com>
Date:	Sat, 12 Mar 2016 16:08:09 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Borislav Petkov <bp@...en8.de>,
	Bryan O'Donoghue <pure.logic@...us-software.ie>,
	Andy Lutomirski <luto@...capital.net>,
	Andy Shevchenko <andy.shevchenko@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	Fenghua Yu <fenghua.yu@...el.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"Yu, Yu-cheng" <yu-cheng.yu@...el.com>
Subject: Re: [PATCH] x86/FPU: Fix FPU handling on legacy FPU machines


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Fri, Mar 11, 2016 at 3:32 AM, Borislav Petkov <bp@...en8.de> wrote:
> > 486 cores like Intel Quark support only the very old, legacy x87 FPU
> > (FSAVE/FRSTOR, CPUID bit FXSR is not set). And our FPU code wasn't
> > handling the saving and restoring there properly. First, Andy Shevchenko
> > reported a splat:
> >
> >   WARNING: CPU: 0 PID: 823 at arch/x86/include/asm/fpu/internal.h:163 fpu__clear+0x8c/0x160
> >
> > which was us trying to execute FXRSTOR on those machines even though
> > they don't support it.
> >
> > After taking care of that, Bryan O'Donoghue reported that a simple FPU
> > test still failed because we weren't initializing the FPU state properly
> > on those machines.
> 
> Obvious Ack to the patch, along with a "how did this ever work
> before?" comment..

So the window for 'real' breakage was relatively short: this is an older bug but 
only became a serious bug with the following upcoming commit:

  58122bf1d856 x86/fpu: Default eagerfpu=on on all CPUs

... but it's nice to have it fixed nevertheless!

Thanks,

	Ingo