arm64: KVM: Do not corrupt registers on failed 64bit CP read

If we fail to emulate a mrrc instruction, we:
1) deliver an exception,
2) spit a nastygram on the console,
3) write back some garbage to Rt/Rt2

While 1) and 2) are perfectly acceptable, 3) is out of the scope of
the architecture... Let's mimick the code in kvm_handle_cp_32 and
be more cautious.
Reviewed-by: Christoffer Dall <cdall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <cdall@linaro.org>

arch/arm64/kvm/sys_regs.c

@@ -1678,20 +1678,25 @@ static int kvm_handle_cp_64(struct kvm_vcpu *vcpu,
 		params.regval |= vcpu_get_reg(vcpu, Rt2) << 32;
 	}
 
-	if (!emulate_cp(vcpu, &params, target_specific, nr_specific))
-		goto out;
-	if (!emulate_cp(vcpu, &params, global, nr_global))
-		goto out;
-
-	unhandled_cp_access(vcpu, &params);
-
-out:
+	/*
+	 * Try to emulate the coprocessor access using the target
+	 * specific table first, and using the global table afterwards.
+	 * If either of the tables contains a handler, handle the
+	 * potential register operation in the case of a read and return
+	 * with success.
+	 */
+	if (!emulate_cp(vcpu, &params, target_specific, nr_specific) ||
+	    !emulate_cp(vcpu, &params, global, nr_global)) {
 		/* Split up the value between registers for the read side */
 		if (!params.is_write) {
 			vcpu_set_reg(vcpu, Rt, lower_32_bits(params.regval));
 			vcpu_set_reg(vcpu, Rt2, upper_32_bits(params.regval));
 		}
 
+		return 1;
+	}
+
+	unhandled_cp_access(vcpu, &params);
 	return 1;
 }