x86/entry, sched/x86: Don't save/restore EFLAGS on task switch

Now that NT is filtered by the SYSENTER entry code, it is safe to skip saving and
restoring flags on task switch.  Also remove a leftover reset of flags on 64-bit
fork.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Acked-by: Andy Lutomirski <luto@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Borislav Petkov <bp@suse.de>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1462416278-11974-2-git-send-email-brgerst@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/entry/entry_32.S

@@ -209,8 +209,6 @@ ENTRY(ret_from_fork)
 	call	schedule_tail
 	GET_THREAD_INFO(%ebp)
 	popl	%eax
-	pushl	$0x0202				# Reset kernel eflags
-	popfl
 
 	/* When we fork, we trace the syscall return in the child, too. */
 	movl    %esp, %eax
@@ -223,8 +221,6 @@ ENTRY(ret_from_kernel_thread)
 	call	schedule_tail
 	GET_THREAD_INFO(%ebp)
 	popl	%eax
-	pushl	$0x0202				# Reset kernel eflags
-	popfl
 	movl	PT_EBP(%esp), %eax
 	call	*PT_EBX(%esp)
 	movl	$0, PT_EAX(%esp)

arch/x86/entry/entry_64.S

@@ -372,9 +372,6 @@ END(ptregs_\func)
 ENTRY(ret_from_fork)
 	LOCK ; btr $TIF_FORK, TI_flags(%r8)
 
-	pushq	$0x0002
-	popfq					/* reset kernel eflags */
-
 	call	schedule_tail			/* rdi: 'prev' task parameter */
 
 	testb	$3, CS(%rsp)			/* from kernel_thread? */

arch/x86/include/asm/switch_to.h

@@ -39,8 +39,7 @@ do {									\
 	 */								\
 	unsigned long ebx, ecx, edx, esi, edi;				\
 									\
-	asm volatile("pushfl\n\t"		/* save    flags */	\
-		     "pushl %%ebp\n\t"		/* save    EBP   */	\
+	asm volatile("pushl %%ebp\n\t"		/* save    EBP   */	\
 		     "movl %%esp,%[prev_sp]\n\t"	/* save    ESP   */ \
 		     "movl %[next_sp],%%esp\n\t"	/* restore ESP   */ \
 		     "movl $1f,%[prev_ip]\n\t"	/* save    EIP   */	\
@@ -49,7 +48,6 @@ do {									\
 		     "jmp __switch_to\n"	/* regparm call  */	\
 		     "1:\t"						\
 		     "popl %%ebp\n\t"		/* restore EBP   */	\
-		     "popfl\n"			/* restore flags */	\
 									\
 		     /* output parameters */				\
 		     : [prev_sp] "=m" (prev->thread.sp),		\