sched/x86_64: Don't save flags on context switch

Now that we have objtool validating AC=1 state for all x86_64 code,
we can once again guarantee clean flags on schedule.

Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Josh Poimboeuf <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/entry/entry_64.S

···
 	pushq	%r13
 	pushq	%r14
 	pushq	%r15
-	pushfq
 
 	/* switch stack */
 	movq	%rsp, TASK_threadsp(%rdi)
···
 #endif
 
 	/* restore callee-saved registers */
-	popfq
 	popq	%r15
 	popq	%r14
 	popq	%r13

arch/x86/include/asm/switch_to.h

···
  * order of the fields must match the code in __switch_to_asm().
  */
 struct inactive_task_frame {
-	unsigned long flags;
 #ifdef CONFIG_X86_64
 	unsigned long r15;
 	unsigned long r14;
 	unsigned long r13;
 	unsigned long r12;
 #else
+	unsigned long flags;
 	unsigned long si;
 	unsigned long di;
 #endif

arch/x86/kernel/process_64.c

···
 	fork_frame = container_of(childregs, struct fork_frame, regs);
 	frame = &fork_frame->frame;
 
-	/*
-	 * For a new task use the RESET flags value since there is no before.
-	 * All the status flags are zero; DF and all the system flags must also
-	 * be 0, specifically IF must be 0 because we context switch to the new
-	 * task with interrupts disabled.
-	 */
-	frame->flags = X86_EFLAGS_FIXED;
 	frame->bp = 0;
 	frame->ret_addr = (unsigned long) ret_from_fork;
 	p->thread.sp = (unsigned long) fork_frame;