KVM: arm64: pkvm: Report optional ID register traps with a 0x18 syndrome

With FEAT_IDST, unimplemented system registers in the feature ID space
must be reported using EC=0x18 at the closest handling EL, rather than
with an UNDEF.

Most of these system registers are always implemented thanks to their
dependency on FEAT_AA64, except for a set of (currently) three registers:
GMID_EL1 (depending on MTE2), CCSIDR2_EL1 (depending on FEAT_CCIDX),
and SMIDR_EL1 (depending on SME).

For these three registers, report their trap as EC=0x18 if they
end-up trapping into KVM and that FEAT_IDST is implemented in the guest.
Otherwise, just make them UNDEF.

Link: https://patch.msgid.link/20260108173233.2911955-9-maz@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>

Author: Marc Zyngier

arch/arm64/kvm/hyp/nvhe/sys_regs.c

@@ -347,6 +347,18 @@ static bool pvm_gic_read_sre(struct kvm_vcpu *vcpu,
 	return true;
 }
 
+static bool pvm_idst_access(struct kvm_vcpu *vcpu,
+			    struct sys_reg_params *p,
+			    const struct sys_reg_desc *r)
+{
+	if (kvm_has_feat(vcpu->kvm, ID_AA64MMFR2_EL1, IDS, IMP))
+		inject_sync64(vcpu, kvm_vcpu_get_esr(vcpu));
+	else
+		inject_undef64(vcpu);
+
+	return false;
+}
+
 /* Mark the specified system register as an AArch32 feature id register. */
 #define AARCH32(REG) { SYS_DESC(REG), .access = pvm_access_id_aarch32 }
 
@@ -477,6 +489,9 @@ static const struct sys_reg_desc pvm_sys_reg_descs[] = {
 
 	HOST_HANDLED(SYS_CCSIDR_EL1),
 	HOST_HANDLED(SYS_CLIDR_EL1),
+	{ SYS_DESC(SYS_CCSIDR2_EL1), .access = pvm_idst_access },
+	{ SYS_DESC(SYS_GMID_EL1), .access = pvm_idst_access },
+	{ SYS_DESC(SYS_SMIDR_EL1), .access = pvm_idst_access },
 	HOST_HANDLED(SYS_AIDR_EL1),
 	HOST_HANDLED(SYS_CSSELR_EL1),
 	HOST_HANDLED(SYS_CTR_EL0),