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) {
...
__fpregs_activate(new_fpu);
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.
Yeah.
--
Regards/Gruss,
Boris.
ECO tip #101: Trim your mails when you reply.
