x86/fpu: Add kernel_fpu_disabled()

Instead of open-coded in_kernel_fpu access, Use kernel_fpu_disabled() in
interrupted_kernel_fpu_idle(), matching the other kernel_fpu_*() methods.

Also add some documentation for in_kernel_fpu.
Reviewed-by: Borislav Petkov <bp@alien8.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/fpu/core.c

@@ -7,6 +7,17 @@
  */
 #include <asm/fpu-internal.h>
 
+/*
+ * Track whether the kernel is using the FPU state
+ * currently.
+ *
+ * This flag is used:
+ *
+ *   - by IRQ context code to potentially use the FPU
+ *     if it's unused.
+ *
+ *   - to debug kernel_fpu_begin()/end() correctness
+ */
 static DEFINE_PER_CPU(bool, in_kernel_fpu);
 
 static void kernel_fpu_disable(void)
@@ -21,6 +32,11 @@ static void kernel_fpu_enable(void)
 	this_cpu_write(in_kernel_fpu, false);
 }
 
+static bool kernel_fpu_disabled(void)
+{
+	return this_cpu_read(in_kernel_fpu);
+}
+
 /*
  * Were we in an interrupt that interrupted kernel mode?
  *
@@ -35,7 +51,7 @@ static void kernel_fpu_enable(void)
  */
 static bool interrupted_kernel_fpu_idle(void)
 {
-	if (this_cpu_read(in_kernel_fpu))
+	if (kernel_fpu_disabled())
 		return false;
 
 	if (use_eager_fpu())