Commit dcd79ed4 authored by Fuad Tabba's avatar Fuad Tabba Committed by Marc Zyngier
Browse files

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: default avatarFuad Tabba <tabba@google.com>
Link: https://patch.msgid.link/20260202085721.3954942-2-tabba@google.com


Signed-off-by: default avatarMarc Zyngier <maz@kernel.org>
parent 9ace4753

arch/arm64/include/asm/kvm_host.h

+0 −3
Original line number Diff line number Diff line
@@ -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

+8 −42
Original line number Diff line number Diff line
@@ -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);

}