Re: [PATCH] x86/FPU: Fix FPU handling on legacy FPU machines

From: Ingo Molnar
Date: Sat Mar 12 2016 - 10:17:33 EST

* Borislav Petkov <bp@xxxxxxxxx> wrote:

> On Fri, Mar 11, 2016 at 10:32:43AM -0800, Linus Torvalds wrote:
> > Obvious Ack to the patch, along with a "how did this ever work
> > before?" comment..
> I had a sarcastic sentence in the commit message which I deleted later:
> "Apparently no one had tried the kernel on a 486er after the FPU
> rewrite. Backwards compatibility is overrated."
> :-)
> I'm still wondering, though, why didn't the Quark people scream
> earlier... And who knows, it was probably b0rked even before the
> FPU rewrite.

So I actually tested legacy FPU support during (and after) the big FPU rewrite - I
even tried FPU emu and we made that work (it was broken for like years).

This particular breakage was made prominent recently, with switching old FPU CPUs
over to eagerfpu context switching:

58122bf1d856 x86/fpu: Default eagerfpu=on on all CPUs

If you check the fix, it only affects eagerfpu code paths. So to see the bug with
Quark (or ancient CPUs) you'd have to have booted it with eagerfpu=on - and most
people avoid twiddling boot parameters if they can avoid it.