KVM: x86: Emulator should set DR6 upon GD like real CPU

It should clear B0-B3 and set BD.
Signed-off-by: NNadav Amit <namit@cs.technion.ac.il>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/emulate.c

@@ -3588,8 +3588,15 @@ static int check_dr_read(struct x86_emulate_ctxt *ctxt)
 	if ((cr4 & X86_CR4_DE) && (dr == 4 || dr == 5))
 		return emulate_ud(ctxt);
 
-	if (check_dr7_gd(ctxt))
+	if (check_dr7_gd(ctxt)) {
+		ulong dr6;
+
+		ctxt->ops->get_dr(ctxt, 6, &dr6);
+		dr6 &= ~15;
+		dr6 |= DR6_BD | DR6_RTM;
+		ctxt->ops->set_dr(ctxt, 6, dr6);
 		return emulate_db(ctxt);
+	}
 
 	return X86EMUL_CONTINUE;
 }