Message-ID: <55D1BE89.1030108@redhat.com>
Date: Mon, 17 Aug 2015 12:59:21 +0200
From: Denys Vlasenko <dvlasenk@redhat.com>
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To: Ingo Molnar <mingo@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
CC: Juergen Gross <jgross@suse.com>, Andy Lutomirski <luto@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, "H. Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Brian Gerst <brgerst@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [GIT PULL] x86 fixes
References: <20150814071500.GA2678@gmail.com> <CA+55aFwP6gJ5Qh7btvf_gME35nvR-se==MuRcvctDqdxEQ9SJA@mail.gmail.com> <20150817080143.GA16928@gmail.com>
In-Reply-To: <20150817080143.GA16928@gmail.com>
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On 08/17/2015 10:01 AM, Ingo Molnar wrote:
> 
> (Sorry about the late reply, wasn't around on the weekend.)
> 
> * Linus Torvalds <torvalds@linux-foundation.org> wrote:
> 
>> Now that said, I doubt anybody cares. Since we don't support the original 80386, 
>> the only way to ever trigger FP emulation is by having a 486SX or possibly a 
>> couple of even rarer clone chips. [...]
> 
> Yeah. So when I re-wrote the FPU code I tried to test math-emu by booting with 
> 'no387': it turned out that ever since the XSAVE code got merged upstream, 
> math-emu oopsed reliably during bootup with a NULL reference, because it wasn't 
> updated to the dynamic allocation logic in:
> 
>   61c4628b5386 ("x86, fpu: split FPU state from task struct - v5")
> 
> That was 6 years ago, so anything v2.6.26 and later probably has 100% non-working 
> math-emu.
> 
> So when I re-introduced static allocations math-emu started working again, to a 
> limited degree: on a modern distro, trying to boot /bin/bash I got a prompt, but 
> various programs would segfault. I did not investigate it any deeper, I suppose 
> the FPU emulation does not go far enough for modern user-space, or maybe it has 
> more bugs.
> 
> So in reality nobody has cared about x86 math-emu in the last 6 years

Well... there is this completely crazy x86 javascript emulator by Fabrice Bellard:

http://bellard.org/jslinux/tech.html

"The CPU is close to a 486 compatible x86 without FPU. The lack of FPU
is not a problem when running Linux as Operating System because it
contains a FPU emulator."


I have it running linux 2.6.20 and busybox here:

http://busybox.net/live_bbox/live_bbox.html

(or rather, *you* will have it running linux 2.6.20 inside your browser,
after you click on that link)
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