KVM: arm64: Add missing hyp_enter when trapping sysreg

Add a missing hypervisor event call for hyp_enter on sysreg trapping,
causing an unbalanced hyp_enter/hyp_exit.

The enum hyp_enter_exit_reason is not ABI, so we can keep the ERET
reasons at the end for clarity.

Fixes: 696dfec22b ("KVM: arm64: Add hyp_enter/hyp_exit events to nVHE/pKVM hyp")
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Tested-by: Fuad Tabba <tabba@google.com>
Link: https://patch.msgid.link/20260617095238.1530121-1-vdonnefort@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>

arch/arm64/include/asm/kvm_hypevents.h

@@ -12,6 +12,7 @@
 enum hyp_enter_exit_reason {
 	HYP_REASON_SMC,
 	HYP_REASON_HVC,
+	HYP_REASON_SYS,
 	HYP_REASON_PSCI,
 	HYP_REASON_HOST_ABORT,
 	HYP_REASON_GUEST_EXIT,

arch/arm64/kvm/hyp/nvhe/hyp-main.c

@@ -932,6 +932,7 @@ void handle_trap(struct kvm_cpu_context *host_ctxt)
 		handle_host_mem_abort(host_ctxt);
 		break;
 	case ESR_ELx_EC_SYS64:
+		trace_hyp_enter(host_ctxt, HYP_REASON_SYS);
 		if (handle_host_mte(esr))
 			break;
 		fallthrough;

arch/arm64/kvm/hyp_trace.c

@@ -409,6 +409,7 @@ static const char *__hyp_enter_exit_reason_str(u8 reason)
 	static const char strs[][12] = {
 		"smc",
 		"hvc",
+		"sys",
 		"psci",
 		"host_abort",
 		"guest_exit",