KVM: arm64: Use standard seq_file iterator for idregs debugfs

The current implementation uses `idreg_debugfs_iter` in `struct
kvm_arch` to track the sequence position. This effectively makes the
iterator shared across all open file descriptors for the VM.

This approach has significant drawbacks:
- It enforces mutual exclusion, preventing concurrent reads of the
  debugfs file (returning -EBUSY).
- It relies on storing transient iterator state in the long-lived VM
  structure (`kvm_arch`).
- The use of `u8` for the iterator index imposes an implicit limit of
  255 registers. While not currently exceeded, this is fragile against
  future architectural growth. Switching to `loff_t` eliminates this
  overflow risk.

Refactor the implementation to use the standard `seq_file` iterator.
Instead of storing state in `kvm_arch`, rely on the `pos` argument
passed to the `start` and `next` callbacks, which tracks the logical
index specific to the file descriptor.

This change enables concurrent access and eliminates the
`idreg_debugfs_iter` field from `struct kvm_arch`.

Signed-off-by: Fuad Tabba <tabba@google.com>
Link: https://patch.msgid.link/20260202085721.3954942-2-tabba@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>

Author: Fuad Tabba

arch/arm64/include/asm/kvm_host.h

@@ -373,9 +373,6 @@ struct kvm_arch {
 	/* Maximum number of counters for the guest */
 	u8 nr_pmu_counters;
 
-	/* Iterator for idreg debugfs */
-	u8	idreg_debugfs_iter;
-
 	/* Hypercall features firmware registers' descriptor */
 	struct kvm_smccc_features smccc_feat;
 	struct maple_tree smccc_filter;

arch/arm64/kvm/sys_regs.c

@@ -4992,7 +4992,7 @@ 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)
+static const struct sys_reg_desc *idregs_debug_find(struct kvm *kvm, loff_t pos)
 {
 	unsigned long i, idreg_idx = 0;
 
@@ -5002,10 +5002,8 @@ static const struct sys_reg_desc *idregs_debug_find(struct kvm *kvm, u8 pos)
 		if (!is_vm_ftr_id_reg(reg_to_encoding(r)))
 			continue;
 
-		if (idreg_idx == pos)
+		if (idreg_idx++ == pos)
 			return r;
-
-		idreg_idx++;
 	}
 
 	return NULL;
@@ -5014,23 +5012,11 @@ static const struct sys_reg_desc *idregs_debug_find(struct kvm *kvm, u8 pos)
 static void *idregs_debug_start(struct seq_file *s, loff_t *pos)
 {
 	struct kvm *kvm = s->private;
-	u8 *iter;
-
-	mutex_lock(&kvm->arch.config_lock);
-
-	iter = &kvm->arch.idreg_debugfs_iter;
-	if (test_bit(KVM_ARCH_FLAG_ID_REGS_INITIALIZED, &kvm->arch.flags) &&
-	    *iter == (u8)~0) {
-		*iter = *pos;
-		if (!idregs_debug_find(kvm, *iter))
-			iter = NULL;
-	} else {
-		iter = ERR_PTR(-EBUSY);
-	}
 
-	mutex_unlock(&kvm->arch.config_lock);
+	if (!test_bit(KVM_ARCH_FLAG_ID_REGS_INITIALIZED, &kvm->arch.flags))
+		return NULL;
 
-	return iter;
+	return (void *)idregs_debug_find(kvm, *pos);
 }
 
 static void *idregs_debug_next(struct seq_file *s, void *v, loff_t *pos)
@@ -5039,37 +5025,19 @@ static void *idregs_debug_next(struct seq_file *s, void *v, loff_t *pos)
 
 	(*pos)++;
 
-	if (idregs_debug_find(kvm, kvm->arch.idreg_debugfs_iter + 1)) {
-		kvm->arch.idreg_debugfs_iter++;
-
-		return &kvm->arch.idreg_debugfs_iter;
-	}
-
-	return NULL;
+	return (void *)idregs_debug_find(kvm, *pos);
 }
 
 static void idregs_debug_stop(struct seq_file *s, void *v)
 {
-	struct kvm *kvm = s->private;
-
-	if (IS_ERR(v))
-		return;
-
-	mutex_lock(&kvm->arch.config_lock);
-
-	kvm->arch.idreg_debugfs_iter = ~0;
-
-	mutex_unlock(&kvm->arch.config_lock);
 }
 
 static int idregs_debug_show(struct seq_file *s, void *v)
 {
-	const struct sys_reg_desc *desc;
+	const struct sys_reg_desc *desc = v;
 	struct kvm *kvm = s->private;
 
-	desc = idregs_debug_find(kvm, kvm->arch.idreg_debugfs_iter);
-
-	if (!desc->name)
+	if (!desc)
 		return 0;
 
 	seq_printf(s, "%20s:\t%016llx\n",
@@ -5089,8 +5057,6 @@ DEFINE_SEQ_ATTRIBUTE(idregs_debug);
 
 void kvm_sys_regs_create_debugfs(struct kvm *kvm)
 {
-	kvm->arch.idreg_debugfs_iter = ~0;
-
 	debugfs_create_file("idregs", 0444, kvm->debugfs_dentry, kvm,
 			    &idregs_debug_fops);
 }