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

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Message-ID: <20160312151720.GB7015@gmail.com>
Date:	Sat, 12 Mar 2016 16:17:20 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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


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

> On Fri, Mar 11, 2016 at 10:32:43AM -0800, Linus Torvalds wrote:
> > Obvious Ack to the patch, along with a "how did this ever work
> > before?" comment..
> 
> I had a sarcastic sentence in the commit message which I deleted later:
> 
> "Apparently no one had tried the kernel on a 486er after the FPU
> rewrite. Backwards compatibility is overrated."
> 
> :-)
> 
> I'm still wondering, though, why didn't the Quark people scream
> earlier... And who knows, it was probably b0rked even before the
> FPU rewrite.

So I actually tested legacy FPU support during (and after) the big FPU rewrite - I 
even tried FPU emu and we made that work (it was broken for like years).

This particular breakage was made prominent recently, with switching old FPU CPUs 
over to eagerfpu context switching:

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

If you check the fix, it only affects eagerfpu code paths. So to see the bug with 
Quark (or ancient CPUs) you'd have to have booted it with eagerfpu=on - and most 
people avoid twiddling boot parameters if they can avoid it.

Thanks,

	Ingo