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

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Message-ID: <CALCETrWGr7JOhR_+Jsx3PS74ALcnZNNEWsJ-EzW09o0uu1ZeFg@mail.gmail.com>
Date:	Mon, 17 Aug 2015 17:19:10 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
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

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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