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KVM: arm64: Use standard seq_file iterator for idregs debugfs

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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>
This commit is contained in:
Fuad Tabba 2026-02-02 08:57:19 +00:00 committed by Marc Zyngier
parent 9ace4753a5
commit dcd79ed450
2 changed files with 8 additions and 45 deletions
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3
arch/arm64/include/asm/kvm_host.h
View File

@ -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;

50
arch/arm64/kvm/sys_regs.c
View File

@ -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);
if (!test_bit(KVM_ARCH_FLAG_ID_REGS_INITIALIZED, &kvm->arch.flags))
return NULL;
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);
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);
}