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

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Date:	Wed, 19 Aug 2015 23:54:35 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Ingo Molnar <mingo@...nel.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

Yes, and MMX, SSE et al didn't have the envision trap support, so you would have to do a full decide and emulation inside the #UD handler.  However, the trap overhead for a lot of those instructions is extreme, as compared to the rather heavyweight x87 instructions (in terms of the ratio between the trap overhead and the actual emulation code.)

On August 19, 2015 3:33:50 PM PDT, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>On Wed, Aug 19, 2015 at 3:00 AM, H. Peter Anvin <hpa@...or.com> wrote:
>>
>> And I bet if CPUID actually reported the right thing it probably
>would work
>> okay.  As I said, I tested this under Qemu which reported an accurate
>(lack
>> of) CPUID for a 486SX.
>
>While I agree that cpuid is a problem for FPU emulation on modern
>CPU's, if this is due to "fucomip" then I think it's just that modern
>distributions (and not-so-modern ones, for that matter) are compiled
>with i686 support, so gcc just generates fucomip directly. So it's
>just "plain FPU" code (no mmx, nothing like that), but it still fails.
>
>The "set regular integer flags instructions" versions of floating
>point compares are some of the bigger improvements to the legacy i87
>instruction set, because the sequences to do FP compares without them
>are just insane. I forget the details, but it's something like "store
>status word to ax, then use sahf to get it into the flags register".
>Crazy crazy crap. So it's no wonder that gcc wants to use a i686-only
>instruction even for just regular FP code if at all possible.
>
>I suspect it shouldn't be that hard to add f[u]compi[p] support to the
>emulator.
>
>But I also expect that most modern distributions are likely fairly
>eager to use mmx etc, which sounds like a major pain to emulate.
>
>                Linus

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