KVM: arm64: Start handling SMCs from EL1

Whelp, the architecture gods have spoken and confirmed that the function
ID space is common between SMCs and HVCs. Not only that, the expectation
is that hypervisors handle calls to both SMC and HVC conduits. KVM
recently picked up support for SMCCCs in commit bd36b1a9 ("KVM:
arm64: nv: Handle SMCs taken from virtual EL2") but scoped it only to a
nested hypervisor.

Let's just open the floodgates and let EL1 access our SMCCC
implementation with the SMC instruction as well.
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230404154050.2270077-6-oliver.upton@linux.dev

arch/arm64/kvm/handle_exit.c

@@ -72,13 +72,15 @@ static int handle_smc(struct kvm_vcpu *vcpu)
 	 *
 	 * We need to advance the PC after the trap, as it would
 	 * otherwise return to the same address...
-	 *
-	 * Only handle SMCs from the virtual EL2 with an immediate of zero and
-	 * skip it otherwise.
 	 */
-	if (!vcpu_is_el2(vcpu) || kvm_vcpu_hvc_get_imm(vcpu)) {
+	kvm_incr_pc(vcpu);
+
+	/*
+	 * SMCs with a nonzero immediate are reserved according to DEN0028E 2.9
+	 * "SMC and HVC immediate value".
+	 */
+	if (kvm_vcpu_hvc_get_imm(vcpu)) {
 		vcpu_set_reg(vcpu, 0, ~0UL);
-		kvm_incr_pc(vcpu);
 		return 1;
 	}
 
@@ -93,8 +95,6 @@ static int handle_smc(struct kvm_vcpu *vcpu)
 	if (ret < 0)
 		vcpu_set_reg(vcpu, 0, ~0UL);
 
-	kvm_incr_pc(vcpu);
-
 	return ret;
 }