Re: Got FPU related warning on Intel Quark during boot

[Bryan O'Donoghue]

On Thu, 2016-03-10 at 17:31 -0800, Andy Lutomirski wrote:
> On Thu, Mar 10, 2016 at 7:45 AM, Bryan O'Donoghue
> <pure.logic@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxx>
> > > 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@xxxxxxxxx>
> > > 
> > > >  }
> > > > 
> > > >  /*
> > > 
> > > 
> > > 
> > 
> > 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

It's the same binary yes.