KVM: arm64: Make idregs debugfs iterator search sysreg table directly

CTR_EL0 complicates the existing scheme for iterating feature ID
registers, as it is not in the contiguous range that we presently
support. Just search the sysreg table for the Nth feature ID register in
anticipation of this. Yes, the debugfs interface has quadratic time
completixy now. Boo hoo.
Reviewed-by: Sebastian Ott <sebott@redhat.com>
Link: https://lore.kernel.org/r/20240619174036.483943-3-oliver.upton@linux.devSigned-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/kvm/sys_regs.c

@@ -2753,8 +2753,6 @@ static struct sys_reg_desc sys_insn_descs[] = {
 	{ SYS_DESC(SYS_DC_CIGDSW), access_dcgsw },
 };
 
-static const struct sys_reg_desc *first_idreg;
-
 static bool trap_dbgdidr(struct kvm_vcpu *vcpu,
 			struct sys_reg_params *p,
 			const struct sys_reg_desc *r)
@@ -3440,6 +3438,25 @@ static bool emulate_sys_reg(struct kvm_vcpu *vcpu,
 	return false;
 }
 
+static const struct sys_reg_desc *idregs_debug_find(struct kvm *kvm, u8 pos)
+{
+	unsigned long i, idreg_idx = 0;
+
+	for (i = 0; i < ARRAY_SIZE(sys_reg_descs); i++) {
+		const struct sys_reg_desc *r = &sys_reg_descs[i];
+
+		if (!is_vm_ftr_id_reg(reg_to_encoding(r)))
+			continue;
+
+		if (idreg_idx == pos)
+			return r;
+
+		idreg_idx++;
+	}
+
+	return NULL;
+}
+
 static void *idregs_debug_start(struct seq_file *s, loff_t *pos)
 {
 	struct kvm *kvm = s->private;
@@ -3451,7 +3468,7 @@ static void *idregs_debug_start(struct seq_file *s, loff_t *pos)
 	if (test_bit(KVM_ARCH_FLAG_ID_REGS_INITIALIZED, &kvm->arch.flags) &&
 	    *iter == (u8)~0) {
 		*iter = *pos;
-		if (*iter >= KVM_ARM_ID_REG_NUM)
+		if (!idregs_debug_find(kvm, *iter))
 			iter = NULL;
 	} else {
 		iter = ERR_PTR(-EBUSY);
@@ -3468,7 +3485,7 @@ static void *idregs_debug_next(struct seq_file *s, void *v, loff_t *pos)
 
 	(*pos)++;
 
-	if ((kvm->arch.idreg_debugfs_iter + 1) < KVM_ARM_ID_REG_NUM) {
+	if (idregs_debug_find(kvm, kvm->arch.idreg_debugfs_iter + 1)) {
 		kvm->arch.idreg_debugfs_iter++;
 
 		return &kvm->arch.idreg_debugfs_iter;
@@ -3493,10 +3510,10 @@ static void idregs_debug_stop(struct seq_file *s, void *v)
 
 static int idregs_debug_show(struct seq_file *s, void *v)
 {
-	struct kvm *kvm = s->private;
 	const struct sys_reg_desc *desc;
+	struct kvm *kvm = s->private;
 
-	desc = first_idreg + kvm->arch.idreg_debugfs_iter;
+	desc = idregs_debug_find(kvm, kvm->arch.idreg_debugfs_iter);
 
 	if (!desc->name)
 		return 0;
@@ -4115,7 +4132,6 @@ void kvm_init_sysreg(struct kvm_vcpu *vcpu)
 
 int __init kvm_sys_reg_table_init(void)
 {
-	struct sys_reg_params params;
 	bool valid = true;
 	unsigned int i;
 	int ret = 0;
@@ -4136,12 +4152,6 @@ int __init kvm_sys_reg_table_init(void)
 	for (i = 0; i < ARRAY_SIZE(invariant_sys_regs); i++)
 		invariant_sys_regs[i].reset(NULL, &invariant_sys_regs[i]);
 
-	/* Find the first idreg (SYS_ID_PFR0_EL1) in sys_reg_descs. */
-	params = encoding_to_params(SYS_ID_PFR0_EL1);
-	first_idreg = find_reg(&params, sys_reg_descs, ARRAY_SIZE(sys_reg_descs));
-	if (!first_idreg)
-		return -EINVAL;
-
 	ret = populate_nv_trap_config();
 
 	for (i = 0; !ret && i < ARRAY_SIZE(sys_reg_descs); i++)