x86/entry/traps: Clear DR6 early in do_debug() and improve the comment

Leaving any bits set in DR6 on return from a debug exception is
asking for trouble. Prevent it by writing zero right away and
clarify the comment.

Signed-off-by: Andy Lutomirski <[e-mail]>
Cc: Andrew Cooper <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Brian Gerst <[e-mail]>
Cc: Denys Vlasenko <[e-mail]>
Cc: H. Peter Anvin <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Oleg Nesterov <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/kernel/traps.c

···
 	ist_enter(regs);
 
 	get_debugreg(dr6, 6);
+	/*
+	 * The Intel SDM says:
+	 *
+	 *   Certain debug exceptions may clear bits 0-3. The remaining
+	 *   contents of the DR6 register are never cleared by the
+	 *   processor. To avoid confusion in identifying debug
+	 *   exceptions, debug handlers should clear the register before
+	 *   returning to the interrupted task.
+	 *
+	 * Keep it simple: clear DR6 immediately.
+	 */
+	set_debugreg(0, 6);
 
 	/* Filter out all the reserved bits which are preset to 1 */
 	dr6 &= ~DR6_RESERVED;
···
 	/* Catch kmemcheck conditions first of all! */
 	if ((dr6 & DR_STEP) && kmemcheck_trap(regs))
 		goto exit;
-
-	/* DR6 may or may not be cleared by the CPU */
-	set_debugreg(0, 6);
 
 	/* Store the virtualized DR6 value */
 	tsk->thread.debugreg6 = dr6;