Date: Sat, 12 Mar 2016 16:08:09 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>,
        "Bryan O'Donoghue" <pure.logic@nexus-software.ie>,
        Andy Lutomirski <luto@amacapital.net>,
        Andy Shevchenko <andy.shevchenko@gmail.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>, Fenghua Yu <fenghua.yu@intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Oleg Nesterov <oleg@redhat.com>,
        "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
Subject: Re: [PATCH] x86/FPU: Fix FPU handling on legacy FPU machines
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* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Fri, Mar 11, 2016 at 3:32 AM, Borislav Petkov <bp@alien8.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