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

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Date:	Mon, 17 Aug 2015 22:56:35 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	Ingo Molnar <mingo@...nel.org>,
	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

I used a very old userspace, and embedded systems are much more likely to use uclibc than glibc.  However, if they try to use SSE without checking they will break on a hell of a lot more hardware.

On August 17, 2015 5:19:10 PM PDT, Andy Lutomirski <luto@...capital.net> wrote:
>On Mon, Aug 17, 2015 at 5:06 PM, H. Peter Anvin <hpa@...or.com> wrote:
>> 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.
>
>But user space needs to avoid SSE2 and such, I suspect.  In general,
>I'd be surprised if things work well if we emulate the FPU (and set
>CR0.em? I haven't checked out Linux's FPU emulation works) if user
>code sees fancy instruction sets exposed and possibly even OSXSAVE.
>
>None of this matters except for testing, since it's very unlikely that
>any CPU exists that supports XSAVE, XMM, SSE2, etc but uses emulated
>x87.  But if we emulate such a beast, things could break, and I bet
>that's what Ingo's seeing.  (Also, lots of distros target "i686" these
>days, and that might cause its own set of problems.)
>
>--Andy

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