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

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Message-ID: <20150817080143.GA16928@gmail.com>
Date:	Mon, 17 Aug 2015 10:01:43 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Juergen Gross <jgross@...e.com>, Andy Lutomirski <luto@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	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

(Sorry about the late reply, wasn't around on the weekend.)

* Linus Torvalds <torvalds@...ux-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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