Re: [lkp] [x86/fpu] 58122bf1d8: WARNING: CPU: 0 PID: 1 at arch/x86/include/asm/fpu/internal.h:529 fpu__restore+0x28f/0x9ab()

From: Borislav Petkov
Date: Sat Feb 27 2016 - 08:13:50 EST

On Sat, Feb 27, 2016 at 01:02:11PM +0100, Ingo Molnar wrote:
> So I'm wondering, why did this commit:
> 58122bf1d856 x86/fpu: Default eagerfpu=on on all CPUs

Hmm, so looking at switch_fpu_prepare():

* If the task has used the math, pre-load the FPU on xsave processors
* or if the past 5 consecutive context-switches used math.
fpu.preload = static_cpu_has(X86_FEATURE_FPU) &&
new_fpu->fpstate_active &&
(use_eager_fpu() || new_fpu->counter > 5);

and later:

if (old_fpu->fpregs_active) {


/* Don't change CR0.TS if we just switch! */
if (fpu.preload) {

so I can see a possible link between 58122bf1d856 and what we're seeing.

But as I've told you offlist, I couldn't confirm that this commit was
the culprit due to my simulated reproducer. So I'm thinking the 0day
guys have a more reliable one.

> trigger the warning, while it never triggered on CPUs that were already
> eagerfpu=on for years?

That I can't explain... yet.

FWIW, the one time splat I saw, happened on an IVB machine on 32-bit
which has always been eagerfpu=on.

> There must be something we are still missing I think.



ECO tip #101: Trim your mails when you reply.