Re: [GIT PULL] x86 fixes

H. Peter Anvin
2015-08-18T06:00:41Z

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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