KVM: arm64: Handle FEAT_IDST for sysregs without specific handlers

Add a bit of infrastrtcture to triage_sysreg_trap() to handle the
case of registers falling into the Feature ID space that do not
have a local handler.

For these, we can directly apply the FEAT_IDST semantics and inject
an EC=0x18 exception. Otherwise, an UNDEF will do.

Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Reviewed-by: Yuan Yao <yaoyuan@linux.alibaba.com>
Link: https://patch.msgid.link/20260108173233.2911955-5-maz@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>

arch/arm64/kvm/emulate-nested.c

@@ -2588,6 +2588,19 @@ local:
 
 		params = esr_sys64_to_params(esr);
 
+		/*
+		 * This implements the pseudocode UnimplementedIDRegister()
+		 * helper for the purpose of dealing with FEAT_IDST.
+		 */
+		if (in_feat_id_space(&params)) {
+			if (kvm_has_feat(vcpu->kvm, ID_AA64MMFR2_EL1, IDS, IMP))
+				kvm_inject_sync(vcpu, kvm_vcpu_get_esr(vcpu));
+			else
+				kvm_inject_undefined(vcpu);
+
+			return true;
+		}
+
 		/*
 		 * Check for the IMPDEF range, as per DDI0487 J.a,
 		 * D18.3.2 Reserved encodings for IMPLEMENTATION

arch/arm64/kvm/sys_regs.h

@@ -49,6 +49,16 @@ struct sys_reg_params {
 				  .Op2 = ((esr) >> 17) & 0x7,			\
 				  .is_write = !((esr) & 1) })
 
+/*
+ * The Feature ID space is defined as the System register space in AArch64
+ * with op0==3, op1=={0, 1, 3}, CRn==0, CRm=={0-7}, op2=={0-7}.
+ */
+static inline bool in_feat_id_space(struct sys_reg_params *p)
+{
+	return (p->Op0 == 3 && !(p->Op1 & 0b100) && p->Op1 != 2 &&
+		p->CRn == 0 && !(p->CRm & 0b1000));
+}
+
 struct sys_reg_desc {
 	/* Sysreg string for debug */
 	const char *name;