x86/db: Split out dr6/7 handling

DR6/7 should be handled before nmi_enter() is invoked and restore after
nmi_exit() to minimize the exposure.

Split it out into helper inlines and bring it into the correct order.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Acked-by: Andy Lutomirski <luto@kernel.org>
Link: https://lkml.kernel.org/r/20200505135314.808628211@linutronix.de

arch/x86/kernel/hw_breakpoint.c

@@ -464,7 +464,7 @@ static int hw_breakpoint_handler(struct die_args *args)
 {
 	int i, cpu, rc = NOTIFY_STOP;
 	struct perf_event *bp;
-	unsigned long dr7, dr6;
+	unsigned long dr6;
 	unsigned long *dr6_p;
 
 	/* The DR6 value is pointed by args->err */
@@ -479,9 +479,6 @@ static int hw_breakpoint_handler(struct die_args *args)
 	if ((dr6 & DR_TRAP_BITS) == 0)
 		return NOTIFY_DONE;
 
-	get_debugreg(dr7, 7);
-	/* Disable breakpoints during exception handling */
-	set_debugreg(0UL, 7);
 	/*
 	 * Assert that local interrupts are disabled
 	 * Reset the DRn bits in the virtualized register value.
@@ -538,7 +535,6 @@ static int hw_breakpoint_handler(struct die_args *args)
 	    (dr6 & (~DR_TRAP_BITS)))
 		rc = NOTIFY_DONE;
 
-	set_debugreg(dr7, 7);
 	put_cpu();
 
 	return rc;