linux-kernel - Re: [GIT PULL] x86 fixes

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Message-ID: <3019C0BF-A7B4-4058-8F92-A6104FE41E1D@zytor.com>
Date:	Mon, 17 Aug 2015 17:06:46 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Ingo Molnar <mingo@...nel.org>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Juergen Gross <jgross@...e.com>,
	Andy Lutomirski <luto@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Brian Gerst <brgerst@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] x86 fixes

User space does not need to treat for FPU instructions, except for performance reasons, because the kernel emulates the full x87 FPU.  So it is localized to the kernel.

On August 17, 2015 4:59:18 PM PDT, Andy Lutomirski <luto@...capital.net> wrote:
>On Mon, Aug 17, 2015 at 1:01 AM, Ingo Molnar <mingo@...nel.org> wrote:
>> So when I re-introduced static allocations math-emu started working
>again, to a
>> limited degree: on a modern distro, trying to boot /bin/bash I got a
>prompt, but
>> various programs would segfault. I did not investigate it any deeper,
>I suppose
>> the FPU emulation does not go far enough for modern user-space, or
>maybe it has
>> more bugs.
>>
>
>Were you testing with just no387 or did you run a VM with SSE2 and
>such turned off?
>
>There's a *lot* of userspace that incorrectly checks for instructions
>without checking for the state support.  I've filed bugs against
>libgcc for this and they're still not fixed IIRC.
>
>--Andy

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