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Subject: Re: [GIT PULL] x86 fixes
From: "H. Peter Anvin" <hpa@zytor.com>
Date: Mon, 17 Aug 2015 22:59:59 -0700
To: Andy Lutomirski <luto@amacapital.net>
CC: Ingo Molnar <mingo@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Juergen Gross <jgross@suse.com>, Andy Lutomirski <luto@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Brian Gerst <brgerst@gmail.com>, Denys Vlasenko <dvlasenk@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>
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I should make it clear: I was using a 486SX model in Qemu.

On August 17, 2015 5:19:10 PM PDT, Andy Lutomirski <luto@amacapital.net> wrote:
>On Mon, Aug 17, 2015 at 5:06 PM, H. Peter Anvin <hpa@zytor.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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