* Ingo Molnar <[email protected]> wrote:
> * Linus Torvalds <[email protected]> wrote:
>
> > On Fri, Mar 11, 2016 at 3:32 AM, Borislav Petkov <[email protected]> wrote:
> > > 486 cores like Intel Quark support only the very old, legacy x87 FPU
> > > (FSAVE/FRSTOR, CPUID bit FXSR is not set). And our FPU code wasn't
> > > handling the saving and restoring there properly. First, Andy Shevchenko
> > > reported a splat:
> > >
> > > WARNING: CPU: 0 PID: 823 at arch/x86/include/asm/fpu/internal.h:163
> > > fpu__clear+0x8c/0x160
> > >
> > > which was us trying to execute FXRSTOR on those machines even though
> > > they don't support it.
> > >
> > > After taking care of that, Bryan O'Donoghue reported that a simple FPU
> > > test still failed because we weren't initializing the FPU state properly
> > > on those machines.
> >
> > Obvious Ack to the patch, along with a "how did this ever work
> > before?" comment..
>
> So the window for 'real' breakage was relatively short: this is an older bug
> but
> only became a serious bug with the following upcoming commit:
>
> 58122bf1d856 x86/fpu: Default eagerfpu=on on all CPUs
And the reason for that is:
void fpu__clear(struct fpu *fpu)
{
WARN_ON_FPU(fpu != ¤t->thread.fpu); /* Almost certainly an
anomaly */
if (!use_eager_fpu() || !static_cpu_has(X86_FEATURE_FPU)) {
/* FPU state will be reallocated lazily at the first use. */
fpu__drop(fpu);
} else {
if (!fpu->fpstate_active) {
fpu__activate_curr(fpu);
user_fpu_begin();
}
copy_init_fpstate_to_fpregs();
}
}
i.e. we only execute the buggy sequence in the !eager_fpu case - and old FPUs
were
not eager-FPU, which hid the bug.
The other bug:
@@ -134,7 +134,7 @@ static void __init fpu__init_system_gene
* Set up the legacy init FPU context. (xstate init might overwrite this
* with a more modern format, if the CPU supports it.)
*/
- fpstate_init_fxstate(&init_fpstate.fxsave);
+ fpstate_init(&init_fpstate);
was also hidden by the fact that it only affects eagerfpu case - but all
previous
eagerfpu bootups were for post-XSAVE CPUs.
Thanks,
Ingo
