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Subject: Re: [GIT PULL] x86 fixes
From: "H. Peter Anvin" <hpa@zytor.com>
Date: Mon, 17 Aug 2015 14:03:09 -0700
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>,
        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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Let me see when I last treated this... but I thought it was much more recently than that.

On August 17, 2015 1:01:43 AM PDT, Ingo Molnar <mingo@kernel.org> 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
>and we can 
>probably remove it for good. I kept it for nostalgic reasons, but I
>guess using 
>v2.4 kernels ought to be enough for those with nostalgia?
>
>> [...] So it's not like the fact that the code is completely wrong and
>crap 
>> actually *matters*, but I still refuse to pull stuff that seems to be
>so 
>> completely screwed up.
>
>That's true, my bad for merging it!
>
>Any objections against removing all of math-emu in v4.3? This would
>simplify the 
>FPU code in various places beyond math-emu/.
>
>Thanks,
>
>	Ingo

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