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>

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index ac7f970c788302531b2e4275ddbf0de0da4c8514..643a199f6e9e8dc59d523760d7cf15d77f864032 100644 (file)
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/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;

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index c8fd7c6a12a13f294ea4bf92dbd1b23f1c880c49..ef6b76ecfb11ee5ec86c9ef00323a8cd0b9c2100 100644 (file)
--- a/arch/arm64/kvm/sys_regs.c
+++ b/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);
 }