linux-kernel - Re: Got FPU related warning on Intel Quark during boot

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Date:	Thu, 10 Mar 2016 15:45:21 +0000
From:	Bryan O'Donoghue <pure.logic@...us-software.ie>
To:	Andy Shevchenko <andy.shevchenko@...il.com>,
	Borislav Petkov <bp@...en8.de>
Cc:	Ingo Molnar <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"Yu, Yu-cheng" <yu-cheng.yu@...el.com>
Subject: Re: Got FPU related warning on Intel Quark during boot

On Thu, 2016-03-10 at 17:22 +0200, Andy Shevchenko wrote:
> On Thu, Mar 10, 2016 at 4:59 PM, Borislav Petkov <bp@...en8.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@...il.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@...ileo:~# ./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@...ileo:~# ./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

root@...ileo:~# uname -aLinux galileo 4.5.0-rc7-next-20160310+ #185 Thu
Mar 10 15:11:10 GMT 2016 i586 GNU/Linux






View attachment "fpu.c" of type "text/x-csrc" (555 bytes)