Re: Got FPU related warning on Intel Quark during boot

[Andy Lutomirski <luto@amacapital.net>]

On Thu, Mar 10, 2016 at 7:45 AM, Bryan O'Donoghue
<pure.logic@nexus-software.ie> wrote:
> On Thu, 2016-03-10 at 17:22 +0200, Andy Shevchenko wrote:
>> On Thu, Mar 10, 2016 at 4:59 PM, Borislav Petkov <bp@alien8.de>
>> wrote:
>> > On Thu, Mar 10, 2016 at 03:31:43PM +0200, Andy Shevchenko wrote:
>> > > Looks like it lacks that one.
>> > >
>> > > # grep -i fxsr /proc/cpuinfo; echo $?
>> > > 1
>> >
>> > Ok, so looking at where the warning comes from:
>> >
>> > [   14.714533] WARNING: CPU: 0 PID: 823 at
>> > arch/x86/include/asm/fpu/internal.h:163 fpu__clear+0x8c/0x160
>> >
>> > static inline void copy_kernel_to_fxregs(struct fxregs_state *fx)
>> > {
>> >         int err;
>> >
>> >         if (config_enabled(CONFIG_X86_32)) {
>> >                 err = check_insn(fxrstor %[fx], "=m" (*fx), [fx]
>> > "m" (*fx));
>> >                       ^^^^^^^^^^^^^^^^^
>> >         } else {
>> >
>> >         ...
>> >
>> >         /* Copying from a kernel buffer to FPU registers should
>> > never fail: */
>> >         WARN_ON_FPU(err);
>> >
>> >
>> > and the stacktrace is pretty clear:
>> >
>> > flush_thread
>> > > -> fpu__clear(&tsk->thread.fpu);
>> >     |-> we are eager by default here:
>> >
>> >         if (!use_eager_fpu() || !static_cpu_has(X86_FEATURE_FPU)) {
>> >                 /* FPU state will be reallocated lazily at the
>> > first use. */
>> >                 fpu__drop(fpu);
>> >         } else {
>> >
>> >                 --> we're in that branch.
>> >
>> >                 copy_init_fpstate_to_fpregs();
>> >                 |-> copy_kernel_to_fxregs()
>> >
>> >
>> > I think we should use FRSTOR on quark, i.e.,
>> > copy_kernel_to_fregs().
>> >
>> > Does this untested wild guess even work?
>> >
>> > ---
>> > diff --git a/arch/x86/kernel/fpu/core.c
>> > b/arch/x86/kernel/fpu/core.c
>> > index dea8e76d60c6..bbafe5e8a1a6 100644
>> > --- a/arch/x86/kernel/fpu/core.c
>> > +++ b/arch/x86/kernel/fpu/core.c
>> > @@ -474,8 +474,11 @@ static inline void
>> > copy_init_fpstate_to_fpregs(void)
>> >  {
>> >         if (use_xsave())
>> >                 copy_kernel_to_xregs(&init_fpstate.xsave, -1);
>> > -       else
>> > +       else if (static_cpu_has(X86_FEATURE_FXSR))
>> >                 copy_kernel_to_fxregs(&init_fpstate.fxsave);
>> > +       else
>> > +               copy_kernel_to_fregs(&init_fpstate.fsave);
>> > +
>>
>> Obviously redundant line, otherwise it indeed works
>>
>> Tested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
>>
>> >  }
>> >
>> >  /*
>>
>>
>>
>
> It works but user-space FPU is broken; something's wrong with the
> initial state of the FPU regs - it looks as though they aren't being
> properly initialized and FPU context in the signal handler is wrong
> too.
>
> Linux 3.8.7:
> /root@galileo:~# ./fpu
> f is 10.000000 g is 10.100000
> Double value is 0.000000
> Double value is 0.100000
> Double value is 0.200000
> ^Chandler value of variable is 0.300000
> Double value is 0.300000
> Double value is 0.400000
>
> Linux-next + Boris' fix:
> root@galileo:~# ./fpu
> f is -nan g is -nan
> Double value is 0.000000
> Double value is 0.100000
> Double value is 0.200000^C
> handler value of variable is -nan
> Double value is 0.300000
> Double value is 0.400000^Z[1]+  Stopped
>

Just to check: are you running the exact same compiled binary on both
kernels?  Because your test case invokes undefined behavior, and I'm a
bit surprised you get anything sensible from it.  That being said, the
f = -nan part is worrisome.

--Andy