x86/fpu: Fix eager-FPU handling on legacy FPU machines

i486 derived 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 in the 'eagerfpu' case.

So after we made eagerfpu the default for all CPU types:

  58122bf x86/fpu: Default eagerfpu=on on all CPUs

these old FPU designs broke. 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.

Take care of all that.

Reported-and-tested-by: Bryan O'Donoghue <[email protected]>
Reported-by: Andy Shevchenko <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Acked-by: Linus Torvalds <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: Fenghua Yu <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Quentin Casasnovas <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Yu-cheng <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

arch/x86/kernel/fpu/core.c

@@ -409,8 +409,10 @@ 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);
 }
 
 /*

arch/x86/kernel/fpu/init.c

@@ -134,7 +134,7 @@ static void __init fpu__init_system_generic(void)
 	 * 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);
 
 	fpu__init_system_mxcsr();
 }