KVM: arm64: Really propagate PSCI SYSTEM_RESET2 arguments to userspace

Commit d43583b8 ("KVM: arm64: Expose PSCI SYSTEM_RESET2 call to the
guest") hooked up the SYSTEM_RESET2 PSCI call for guests but failed to
preserve its arguments for userspace, instead overwriting them with
zeroes via smccc_set_retval(). As Linux only passes zeroes for these
arguments, this appeared to be working for Linux guests. Oh well.

Don't call smccc_set_retval() for a SYSTEM_RESET2 heading to userspace
and instead set X0 (and only X0) explicitly to PSCI_RET_INTERNAL_FAILURE
just in case the vCPU re-enters the guest.

Fixes: d43583b8 ("KVM: arm64: Expose PSCI SYSTEM_RESET2 call to the guest")
Reported-by: Andrew Walbran <qwandor@google.com>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220309181308.982-1-will@kernel.org

arch/arm64/kvm/psci.c

@@ -367,14 +367,14 @@ static int kvm_psci_1_x_call(struct kvm_vcpu *vcpu, u32 minor)
 		if (minor >= 1) {
 			arg = smccc_get_arg1(vcpu);
 
-			if (arg > PSCI_1_1_RESET_TYPE_SYSTEM_WARM_RESET &&
-			    arg < PSCI_1_1_RESET_TYPE_VENDOR_START) {
-				val = PSCI_RET_INVALID_PARAMS;
-			} else {
+			if (arg <= PSCI_1_1_RESET_TYPE_SYSTEM_WARM_RESET ||
+			    arg >= PSCI_1_1_RESET_TYPE_VENDOR_START) {
 				kvm_psci_system_reset2(vcpu);
-				val = PSCI_RET_INTERNAL_FAILURE;
-				ret = 0;
+				vcpu_set_reg(vcpu, 0, PSCI_RET_INTERNAL_FAILURE);
+				return 0;
 			}
+
+			val = PSCI_RET_INVALID_PARAMS;
 			break;
 		}
 		fallthrough;