x86: split out core __math_state_restore

Split the core fpu state restoration out into __math_state_restore, which
assumes that cr0.TS is clear and that the fpu context has been initialized.

This will be used during context switch.  There are two reasons this is
desireable:

- There's a small clarification.  When __switch_to() calls math_state_restore,
  it relies on the fact that tsk_used_math() returns true, and so will
  never do a blocking init_fpu().  __math_state_restore() does not have
  (or need) that logic, so the question never arises.

- It allows the clts() to be moved earler in __switch_to() so it can be performed
  while cpu context updates are batched (will be done in a later patch).

[ Impact: refactor code to make reuse cleaner; no functional change ]

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Alok Kataria <akataria@vmware.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>

arch/x86/include/asm/i387.h

@@ -26,6 +26,7 @@ extern void fpu_init(void);
 extern void mxcsr_feature_mask_init(void);
 extern int init_fpu(struct task_struct *child);
 extern asmlinkage void math_state_restore(void);
+extern void __math_state_restore(void);
 extern void init_thread_xstate(void);
 extern int dump_fpu(struct pt_regs *, struct user_i387_struct *);
 

arch/x86/kernel/traps.c

@@ -813,6 +813,28 @@ asmlinkage void __attribute__((weak)) smp_threshold_interrupt(void)
 {
 }
 
+/*
+ * __math_state_restore assumes that cr0.TS is already clear and the
+ * fpu state is all ready for use.  Used during context switch.
+ */
+void __math_state_restore(void)
+{
+	struct thread_info *thread = current_thread_info();
+	struct task_struct *tsk = thread->task;
+
+	/*
+	 * Paranoid restore. send a SIGSEGV if we fail to restore the state.
+	 */
+	if (unlikely(restore_fpu_checking(tsk))) {
+		stts();
+		force_sig(SIGSEGV, tsk);
+		return;
+	}
+
+	thread->status |= TS_USEDFPU;	/* So we fnsave on switch_to() */
+	tsk->fpu_counter++;
+}
+
 /*
  * 'math_state_restore()' saves the current math information in the
  * old math state array, and gets the new ones from the current task
@@ -844,17 +866,8 @@ asmlinkage void math_state_restore(void)
 	}
 
 	clts();				/* Allow maths ops (or we recurse) */
-	/*
-	 * Paranoid restore. send a SIGSEGV if we fail to restore the state.
-	 */
-	if (unlikely(restore_fpu_checking(tsk))) {
-		stts();
-		force_sig(SIGSEGV, tsk);
-		return;
-	}
 
-	thread->status |= TS_USEDFPU;	/* So we fnsave on switch_to() */
-	tsk->fpu_counter++;
+	__math_state_restore();
 }
 EXPORT_SYMBOL_GPL(math_state_restore);