Re: [GIT PULL] x86 fixes

Ingo Molnar
2015-08-17T08:01:51Z

(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