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

From: Andy Lutomirski
Date: Sat Mar 12 2016 - 12:22:39 EST

On Mar 11, 2016 2:20 PM, "Borislav Petkov" <bp@xxxxxxxxx> wrote:
> On Fri, Mar 11, 2016 at 02:07:19PM -0800, Dave Hansen wrote:
> > I've actually got 4.0 running on my Quark board. The FPU rewrite
> > dropped in just after that iirc.
> 4.2 or so... Ok, so it looks like we broke it then.

For reference, what are the QEMU options and boot options you used to
trigger this? I'm asking because I tested eagerfpu=on without fxsr a
few weeks ago in QEMU, and I didn't trigger it. Maybe I needed to
force KVM off or something. Off the top of my head, I'm guessing that
when I wrote "x86/fpu: Fix FNSAVE usage in eagerfpu mode", I was
inadvertently using a bastardize combination of FNSAVE and
FXRSTOR-of-init-state, KVM let the FXRSTOR through despite not
advertising it in CPUID, and it papered over the init issue because
the wrong init state format was hidden by using the wrong instruction
to load it.

Sigh. Yet more reason for Intel to add chicken bits to *turn off* new features.