A trace remote relies on ring-buffer remotes to read and control
compatible tracing buffers, written by entity such as firmware or
hypervisor.

Add a Tracefs directory remotes/ that contains all instances of trace
remotes. Each instance follows the same hierarchy as any other to ease
the support by existing user-space tools.

This currently does not provide any event support, which will come
later.

Signed-off-by: Vincent Donnefort <vdonnefort@google.com>

diff --git a/include/linux/trace_remote.h b/include/linux/trace_remote.h
new file mode 100644
index 000000000000..feb3433c2128
--- /dev/null
+++ b/include/linux/trace_remote.h
@@ -0,0 +1,80 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef _LINUX_TRACE_REMOTE_H
+#define _LINUX_TRACE_REMOTE_H
+
+#include <linux/ring_buffer.h>
+
+/**
+ * struct trace_remote_callbacks - Callbacks used by Tracefs to control the remote
+ *
+ * @load_trace_buffer:  Called before Tracefs accesses the trace buffer for the first
+ *			time. Must return a &trace_buffer_desc
+ *			(most likely filled with trace_remote_alloc_buffer())
+ * @unload_trace_buffer:
+ *			Called once Tracefs has no use for the trace buffer
+ *			(most likely call trace_remote_free_buffer())
+ * @enable_tracing:	Called on Tracefs tracing_on. It is expected from the
+ *			remote to allow writing.
+ * @swap_reader_page:	Called when Tracefs consumes a new page from a
+ *			ring-buffer. It is expected from the remote to isolate a
+ *			new reader-page from the @cpu ring-buffer.
+ */
+struct trace_remote_callbacks {
+	struct trace_buffer_desc *(*load_trace_buffer)(unsigned long size, void *priv);
+	void	(*unload_trace_buffer)(struct trace_buffer_desc *desc, void *priv);
+	int	(*enable_tracing)(bool enable, void *priv);
+	int	(*swap_reader_page)(unsigned int cpu, void *priv);
+};
+
+/**
+ * trace_remote_register() - Register a Tracefs remote
+ *
+ * A trace remote is an entity, outside of the kernel (most likely firmware or
+ * hypervisor) capable of writing events into a Tracefs compatible ring-buffer.
+ * The kernel would then act as a reader.
+ *
+ * The registered remote will be found under the Tracefs directory
+ * remotes/<name>.
+ *
+ * @name:	Name of the remote, used for the Tracefs remotes/ directory.
+ * @cbs:	Set of callbacks used to control the remote.
+ * @priv:	Private data, passed to each callback from @cbs.
+ * @events:	Array of events. &remote_event.name and &remote_event.id must be
+ *		filled by the caller.
+ * @nr_events:	Number of events in the @events array.
+ *
+ * Return: 0 on success, negative error code on failure.
+ */
+int trace_remote_register(const char *name, struct trace_remote_callbacks *cbs, void *priv);
+
+/**
+ * trace_remote_alloc_buffer() - Dynamically allocate a trace buffer
+ *
+ * Helper to dynamically allocate a set of pages (enough to cover @buffer_size)
+ * for each CPU from @cpumask and fill @desc. Most likely called from
+ * &trace_remote_callbacks.load_trace_buffer.
+ *
+ * @desc:		Uninitialized trace_buffer_desc
+ * @desc_size:		Size of the trace_buffer_desc. Must be at least equal to
+ *			trace_buffer_desc_size()
+ * @buffer_size:	Size in bytes of each per-CPU ring-buffer
+ * @cpumask:		CPUs to allocate a ring-buffer for
+ *
+ * Return: 0 on success, negative error code on failure.
+ */
+int trace_remote_alloc_buffer(struct trace_buffer_desc *desc, size_t desc_size, size_t buffer_size,
+			      const struct cpumask *cpumask);
+
+/**
+ * trace_remote_free_buffer() - Free trace buffer allocated with
+ *				trace_remote_alloc_buffer()
+ *
+ * Most likely called from &trace_remote_callbacks.unload_trace_buffer.
+ *
+ * @desc:	Descriptor of the per-CPU ring-buffers, originally filled by
+ *		trace_remote_alloc_buffer()
+ */
+void trace_remote_free_buffer(struct trace_buffer_desc *desc);
+
+#endif
diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index bfa2ec46e075..135edf143073 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -1278,4 +1278,7 @@ config HIST_TRIGGERS_DEBUG
 
 source "kernel/trace/rv/Kconfig"
 
+config TRACE_REMOTE
+	bool
+
 endif # FTRACE
diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile
index fc5dcc888e13..6e75d710fc25 100644
--- a/kernel/trace/Makefile
+++ b/kernel/trace/Makefile
@@ -127,4 +127,5 @@ obj-$(CONFIG_FPROBE_EVENTS) += trace_fprobe.o
 obj-$(CONFIG_TRACEPOINT_BENCHMARK) += trace_benchmark.o
 obj-$(CONFIG_RV) += rv/
 
+obj-$(CONFIG_TRACE_REMOTE) += trace_remote.o
 libftrace-y := ftrace.o
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 8bd4ec08fb36..1fcbdea992d2 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -9356,7 +9356,7 @@ static struct dentry *tracing_dentry_percpu(struct trace_array *tr, int cpu)
 	return tr->percpu_dir;
 }
 
-static struct dentry *
+struct dentry *
 trace_create_cpu_file(const char *name, umode_t mode, struct dentry *parent,
 		      void *data, long cpu, const struct file_operations *fops)
 {
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index b6d42fe06115..fdbcd068ba38 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -680,6 +680,12 @@ struct dentry *trace_create_file(const char *name,
 				 struct dentry *parent,
 				 void *data,
 				 const struct file_operations *fops);
+struct dentry *trace_create_cpu_file(const char *name,
+				     umode_t mode,
+				     struct dentry *parent,
+				     void *data,
+				     long cpu,
+				     const struct file_operations *fops);
 
 
 /**
diff --git a/kernel/trace/trace_remote.c b/kernel/trace/trace_remote.c
new file mode 100644
index 000000000000..3d80ff98fd90
--- /dev/null
+++ b/kernel/trace/trace_remote.c
@@ -0,0 +1,570 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2025 - Google LLC
+ * Author: Vincent Donnefort <vdonnefort@google.com>
+ */
+
+#include <linux/kstrtox.h>
+#include <linux/lockdep.h>
+#include <linux/mutex.h>
+#include <linux/tracefs.h>
+#include <linux/trace_remote.h>
+#include <linux/trace_seq.h>
+#include <linux/types.h>
+
+#include "trace.h"
+
+#define TRACEFS_DIR		"remotes"
+#define TRACEFS_MODE_WRITE	0640
+#define TRACEFS_MODE_READ	0440
+
+struct trace_remote_iterator {
+	struct trace_remote		*remote;
+	struct trace_seq		seq;
+	struct delayed_work		poll_work;
+	unsigned long			lost_events;
+	u64				ts;
+	int				cpu;
+	int				evt_cpu;
+};
+
+struct trace_remote {
+	struct trace_remote_callbacks	*cbs;
+	void				*priv;
+	struct trace_buffer		*trace_buffer;
+	struct trace_buffer_desc	*trace_buffer_desc;
+	unsigned long			trace_buffer_size;
+	struct ring_buffer_remote	rb_remote;
+	struct mutex			lock;
+	unsigned int			nr_readers;
+	unsigned int			poll_ms;
+	bool				tracing_on;
+};
+
+static bool trace_remote_loaded(struct trace_remote *remote)
+{
+	return remote->trace_buffer;
+}
+
+static int trace_remote_load(struct trace_remote *remote)
+{
+	struct ring_buffer_remote *rb_remote = &remote->rb_remote;
+
+	lockdep_assert_held(&remote->lock);
+
+	if (trace_remote_loaded(remote))
+		return 0;
+
+	remote->trace_buffer_desc = remote->cbs->load_trace_buffer(remote->trace_buffer_size,
+								   remote->priv);
+	if (IS_ERR(remote->trace_buffer_desc))
+		return PTR_ERR(remote->trace_buffer_desc);
+
+	rb_remote->desc = remote->trace_buffer_desc;
+	rb_remote->swap_reader_page = remote->cbs->swap_reader_page;
+	rb_remote->priv = remote->priv;
+	remote->trace_buffer = ring_buffer_alloc_remote(rb_remote);
+	if (!remote->trace_buffer) {
+		remote->cbs->unload_trace_buffer(remote->trace_buffer_desc, remote->priv);
+		return -ENOMEM;
+	}
+
+	return 0;
+}
+
+static void trace_remote_try_unload(struct trace_remote *remote)
+{
+	lockdep_assert_held(&remote->lock);
+
+	if (!trace_remote_loaded(remote))
+		return;
+
+	/* The buffer is being read or writable */
+	if (remote->nr_readers || remote->tracing_on)
+		return;
+
+	/* The buffer has readable data */
+	if (!ring_buffer_empty(remote->trace_buffer))
+		return;
+
+	ring_buffer_free(remote->trace_buffer);
+	remote->trace_buffer = NULL;
+	remote->cbs->unload_trace_buffer(remote->trace_buffer_desc, remote->priv);
+}
+
+static int trace_remote_enable_tracing(struct trace_remote *remote)
+{
+	int ret;
+
+	lockdep_assert_held(&remote->lock);
+
+	if (remote->tracing_on)
+		return 0;
+
+	ret = trace_remote_load(remote);
+	if (ret)
+		return ret;
+
+	ret = remote->cbs->enable_tracing(true, remote->priv);
+	if (ret) {
+		trace_remote_try_unload(remote);
+		return ret;
+	}
+
+	remote->tracing_on = true;
+
+	return 0;
+}
+
+static int trace_remote_disable_tracing(struct trace_remote *remote)
+{
+	int ret;
+
+	lockdep_assert_held(&remote->lock);
+
+	if (!remote->tracing_on)
+		return 0;
+
+	ret = remote->cbs->enable_tracing(false, remote->priv);
+	if (ret)
+		return ret;
+
+	ring_buffer_poll_remote(remote->trace_buffer, RING_BUFFER_ALL_CPUS);
+	remote->tracing_on = false;
+	trace_remote_try_unload(remote);
+
+	return 0;
+}
+
+static ssize_t
+tracing_on_write(struct file *filp, const char __user *ubuf, size_t cnt, loff_t *ppos)
+{
+	struct trace_remote *remote = filp->private_data;
+	unsigned long val;
+	int ret;
+
+	ret = kstrtoul_from_user(ubuf, cnt, 10, &val);
+	if (ret)
+		return ret;
+
+	guard(mutex)(&remote->lock);
+
+	ret = val ? trace_remote_enable_tracing(remote) : trace_remote_disable_tracing(remote);
+	if (ret)
+		return ret;
+
+	return cnt;
+}
+static int tracing_on_show(struct seq_file *s, void *unused)
+{
+	struct trace_remote *remote = s->private;
+
+	seq_printf(s, "%d\n", remote->tracing_on);
+
+	return 0;
+}
+DEFINE_SHOW_STORE_ATTRIBUTE(tracing_on);
+
+static ssize_t buffer_size_kb_write(struct file *filp, const char __user *ubuf, size_t cnt,
+				    loff_t *ppos)
+{
+	struct trace_remote *remote = filp->private_data;
+	unsigned long val;
+	int ret;
+
+	ret = kstrtoul_from_user(ubuf, cnt, 10, &val);
+	if (ret)
+		return ret;
+
+	/* KiB to Bytes */
+	if (!val || check_shl_overflow(val, 10, &val))
+		return -EINVAL;
+
+	guard(mutex)(&remote->lock);
+
+	remote->trace_buffer_size = val;
+
+	return cnt;
+}
+
+static int buffer_size_kb_show(struct seq_file *s, void *unused)
+{
+	struct trace_remote *remote = s->private;
+
+	seq_printf(s, "%lu (%s)\n", remote->trace_buffer_size >> 10,
+		   trace_remote_loaded(remote) ? "loaded" : "unloaded");
+
+	return 0;
+}
+DEFINE_SHOW_STORE_ATTRIBUTE(buffer_size_kb);
+
+static int trace_remote_get(struct trace_remote *remote, int cpu)
+{
+	int ret;
+
+	if (remote->nr_readers == UINT_MAX)
+		return -EBUSY;
+
+	ret = trace_remote_load(remote);
+	if (ret)
+		return ret;
+
+	remote->nr_readers++;
+
+	return 0;
+}
+
+static void trace_remote_put(struct trace_remote *remote)
+{
+	if (WARN_ON(!remote->nr_readers))
+		return;
+
+	remote->nr_readers--;
+	if (remote->nr_readers)
+		return;
+
+	trace_remote_try_unload(remote);
+}
+
+static void __poll_remote(struct work_struct *work)
+{
+	struct delayed_work *dwork = to_delayed_work(work);
+	struct trace_remote_iterator *iter;
+
+	iter = container_of(dwork, struct trace_remote_iterator, poll_work);
+	ring_buffer_poll_remote(iter->remote->trace_buffer, iter->cpu);
+	schedule_delayed_work((struct delayed_work *)work,
+			      msecs_to_jiffies(iter->remote->poll_ms));
+}
+
+static struct trace_remote_iterator
+*trace_remote_iter(struct trace_remote *remote, int cpu, enum tri_type type)
+{
+	struct trace_remote_iterator *iter = NULL;
+	int ret;
+
+	lockdep_assert_held(&remote->lock);
+
+
+	ret = trace_remote_get(remote, cpu);
+	if (ret)
+		return ERR_PTR(ret);
+
+	/* Test the CPU */
+	ret = ring_buffer_poll_remote(remote->trace_buffer, cpu);
+	if (ret)
+		goto err;
+
+	iter = kzalloc(sizeof(*iter), GFP_KERNEL);
+	if (iter) {
+		iter->remote = remote;
+		iter->cpu = cpu;
+		trace_seq_init(&iter->seq);
+		INIT_DELAYED_WORK(&iter->poll_work, __poll_remote);
+		schedule_delayed_work(&iter->poll_work, msecs_to_jiffies(remote->poll_ms));
+
+		return iter;
+	}
+	ret = -ENOMEM;
+
+err:
+	kfree(iter);
+	trace_remote_put(remote);
+
+	return ERR_PTR(ret);
+}
+
+static void trace_remote_iter_free(struct trace_remote_iterator *iter)
+{
+	struct trace_remote *remote;
+
+	if (!iter)
+		return;
+
+	remote = iter->remote;
+
+	lockdep_assert_held(&remote->lock);
+
+	kfree(iter);
+	trace_remote_put(remote);
+}
+
+static bool trace_remote_iter_read_event(struct trace_remote_iterator *iter)
+{
+	struct trace_buffer *trace_buffer = iter->remote->trace_buffer;
+	int cpu = iter->cpu;
+
+	if (cpu != RING_BUFFER_ALL_CPUS) {
+		if (ring_buffer_empty_cpu(trace_buffer, cpu))
+			return false;
+
+		if (!ring_buffer_peek(trace_buffer, cpu, &iter->ts, &iter->lost_events))
+			return false;
+
+		iter->evt_cpu = cpu;
+		return true;
+	}
+
+	iter->ts = U64_MAX;
+	for_each_possible_cpu(cpu) {
+		unsigned long lost_events;
+		u64 ts;
+
+		if (ring_buffer_empty_cpu(trace_buffer, cpu))
+			continue;
+
+		if (!ring_buffer_peek(trace_buffer, cpu, &ts, &lost_events))
+			continue;
+
+		if (ts >= iter->ts)
+			continue;
+
+		iter->ts = ts;
+		iter->evt_cpu = cpu;
+		iter->lost_events = lost_events;
+	}
+
+	return iter->ts != U64_MAX;
+}
+
+static int trace_remote_iter_print(struct trace_remote_iterator *iter)
+{
+	unsigned long usecs_rem;
+	u64 ts = iter->ts;
+
+	if (iter->lost_events)
+		trace_seq_printf(&iter->seq, "CPU:%d [LOST %lu EVENTS]\n",
+				 iter->evt_cpu, iter->lost_events);
+
+	do_div(ts, 1000);
+	usecs_rem = do_div(ts, USEC_PER_SEC);
+
+	trace_seq_printf(&iter->seq, "[%03d]\t%5llu.%06lu: ", iter->evt_cpu,
+			 ts, usecs_rem);
+
+	return trace_seq_has_overflowed(&iter->seq) ? -EOVERFLOW : 0;
+}
+
+static int trace_pipe_open(struct inode *inode, struct file *filp)
+{
+	struct trace_remote *remote = inode->i_private;
+	struct trace_remote_iterator *iter;
+	int cpu = RING_BUFFER_ALL_CPUS;
+
+	if (inode->i_cdev)
+		cpu = (long)inode->i_cdev - 1;
+
+	guard(mutex)(&remote->lock);
+	iter = trace_remote_iter(remote, cpu);
+	filp->private_data = iter;
+
+	return IS_ERR(iter) ? PTR_ERR(iter) : 0;
+}
+
+static int trace_pipe_release(struct inode *inode, struct file *filp)
+{
+	struct trace_remote_iterator *iter = filp->private_data;
+	struct trace_remote *remote = iter->remote;
+
+	guard(mutex)(&remote->lock);
+
+	trace_remote_iter_free(iter);
+
+	return 0;
+}
+
+static ssize_t trace_pipe_read(struct file *filp, char __user *ubuf, size_t cnt, loff_t *ppos)
+{
+	struct trace_remote_iterator *iter = filp->private_data;
+	struct trace_buffer *trace_buffer = iter->remote->trace_buffer;
+	int ret;
+
+copy_to_user:
+	ret = trace_seq_to_user(&iter->seq, ubuf, cnt);
+	if (ret != -EBUSY)
+		return ret;
+
+	trace_seq_init(&iter->seq);
+
+	ret = ring_buffer_wait(trace_buffer, iter->cpu, 0, NULL, NULL);
+	if (ret < 0)
+		return ret;
+
+	while (trace_remote_iter_read_event(iter)) {
+		int prev_len = iter->seq.seq.len;
+
+		if (trace_remote_iter_print_event(iter)) {
+			iter->seq.seq.len = prev_len;
+			break;
+		}
+
+		ring_buffer_consume(trace_buffer, iter->evt_cpu, NULL, NULL);
+	}
+
+	goto copy_to_user;
+}
+
+static const struct file_operations trace_pipe_fops = {
+	.open		= trace_pipe_open,
+	.read		= trace_pipe_read,
+	.release	= trace_pipe_release,
+};
+
+static int trace_remote_init_tracefs(const char *name, struct trace_remote *remote)
+{
+	struct dentry *remote_d, *percpu_d;
+	static struct dentry *root;
+	static DEFINE_MUTEX(lock);
+	bool root_inited = false;
+	int cpu;
+
+	guard(mutex)(&lock);
+
+	if (!root) {
+		root = tracefs_create_dir(TRACEFS_DIR, NULL);
+		if (!root) {
+			pr_err("Failed to create tracefs dir "TRACEFS_DIR"\n");
+			return -ENOMEM;
+		}
+		root_inited = true;
+	}
+
+	remote_d = tracefs_create_dir(name, root);
+	if (!remote_d) {
+		pr_err("Failed to create tracefs dir "TRACEFS_DIR"%s/\n", name);
+		goto err;
+	}
+
+	if (!trace_create_file("tracing_on", TRACEFS_MODE_WRITE, remote_d, remote,
+			       &tracing_on_fops) ||
+	    !trace_create_file("buffer_size_kb", TRACEFS_MODE_WRITE, remote_d, remote,
+			       &buffer_size_kb_fops) ||
+	    !trace_create_file("trace_pipe", TRACEFS_MODE_READ, remote_d, remote,
+			       &trace_pipe_fops))
+		goto err;
+
+	percpu_d = tracefs_create_dir("per_cpu", remote_d);
+	if (!percpu_d) {
+		pr_err("Failed to create tracefs dir "TRACEFS_DIR"%s/per_cpu/\n", name);
+		goto err;
+	}
+
+	for_each_possible_cpu(cpu) {
+		struct dentry *cpu_d;
+		char cpu_name[16];
+
+		snprintf(cpu_name, sizeof(cpu_name), "cpu%d", cpu);
+		cpu_d = tracefs_create_dir(cpu_name, percpu_d);
+		if (!cpu_d) {
+			pr_err("Failed to create tracefs dir "TRACEFS_DIR"%s/percpu/cpu%d\n",
+			       name, cpu);
+			goto err;
+		}
+
+		if (!trace_create_cpu_file("trace_pipe", TRACEFS_MODE_READ, cpu_d, remote, cpu,
+					   &trace_pipe_fops))
+			goto err;
+	}
+
+	return 0;
+
+err:
+	if (root_inited) {
+		tracefs_remove(root);
+		root = NULL;
+	} else {
+		tracefs_remove(remote_d);
+	}
+
+	return -ENOMEM;
+}
+
+int trace_remote_register(const char *name, struct trace_remote_callbacks *cbs, void *priv)
+{
+	struct trace_remote *remote;
+
+	remote = kzalloc(sizeof(*remote), GFP_KERNEL);
+	if (!remote)
+		return -ENOMEM;
+
+	remote->cbs = cbs;
+	remote->priv = priv;
+	remote->trace_buffer_size = 7 << 10;
+	remote->poll_ms = 100;
+	mutex_init(&remote->lock);
+
+	if (trace_remote_init_tracefs(name, remote)) {
+		kfree(remote);
+		return -ENOMEM;
+	}
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(trace_remote_register);
+
+void trace_remote_free_buffer(struct trace_buffer_desc *desc)
+{
+	struct ring_buffer_desc *rb_desc;
+	int cpu;
+
+	for_each_ring_buffer_desc(rb_desc, cpu, desc) {
+		unsigned int id;
+
+		free_page(rb_desc->meta_va);
+
+		for (id = 0; id < rb_desc->nr_page_va; id++)
+			free_page(rb_desc->page_va[id]);
+	}
+}
+EXPORT_SYMBOL_GPL(trace_remote_free_buffer);
+
+int trace_remote_alloc_buffer(struct trace_buffer_desc *desc, size_t desc_size, size_t buffer_size,
+			      const struct cpumask *cpumask)
+{
+	unsigned int nr_pages = max(DIV_ROUND_UP(buffer_size, PAGE_SIZE), 2UL) + 1;
+	void *desc_end = desc + desc_size;
+	struct ring_buffer_desc *rb_desc;
+	int cpu, ret = -ENOMEM;
+
+	if (desc_size < struct_size(desc, __data, 0))
+		return -EINVAL;
+
+	desc->nr_cpus = 0;
+	desc->struct_len = struct_size(desc, __data, 0);
+
+	rb_desc = (struct ring_buffer_desc *)&desc->__data[0];
+
+	for_each_cpu(cpu, cpumask) {
+		unsigned int id;
+
+		if ((void *)rb_desc + struct_size(rb_desc, page_va, nr_pages) > desc_end) {
+			ret = -EINVAL;
+			goto err;
+		}
+
+		rb_desc->cpu = cpu;
+		rb_desc->nr_page_va = 0;
+		rb_desc->meta_va = (unsigned long)__get_free_page(GFP_KERNEL);
+		if (!rb_desc->meta_va)
+			goto err;
+
+		for (id = 0; id < nr_pages; id++) {
+			rb_desc->page_va[id] = (unsigned long)__get_free_page(GFP_KERNEL);
+			if (!rb_desc->page_va[id])
+				goto err;
+
+			rb_desc->nr_page_va++;
+		}
+		desc->nr_cpus++;
+		desc->struct_len += offsetof(struct ring_buffer_desc, page_va);
+		desc->struct_len += struct_size(rb_desc, page_va, rb_desc->nr_page_va);
+		rb_desc = __next_ring_buffer_desc(rb_desc);
+	}
+
+	return 0;
+
+err:
+	trace_remote_free_buffer(desc);
+	return ret;
+}
+EXPORT_SYMBOL_GPL(trace_remote_alloc_buffer);
-- 
2.52.0.457.g6b5491de43-goog

On Mon, 26 Jan 2026 10:43:54 +0000
Vincent Donnefort <vdonnefort@google.com> wrote:

> +	if (!trace_create_file("tracing_on", TRACEFS_MODE_WRITE, remote_d, remote,
> +			       &tracing_on_fops) ||
> +	    !trace_create_file("buffer_size_kb", TRACEFS_MODE_WRITE, remote_d, remote,
> +			       &buffer_size_kb_fops) ||
> +	    !trace_create_file("trace_pipe", TRACEFS_MODE_READ, remote_d, remote,
> +			       &trace_pipe_fops))
> +		goto err;
> +

Oh, and don't do this in an if statement. Make them each individually as:

	ret = trace_create_file(...);
	if (!ret)
		goto err;

Linus hates complex if statements and I can see him having a conniption if
he were to see this.

-- Steve

On Mon, 26 Jan 2026 10:43:54 +0000
Vincent Donnefort <vdonnefort@google.com> wrote:

> diff --git a/include/linux/trace_remote.h b/include/linux/trace_remote.h
> new file mode 100644
> index 000000000000..feb3433c2128
> --- /dev/null
> +++ b/include/linux/trace_remote.h
> @@ -0,0 +1,80 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef _LINUX_TRACE_REMOTE_H
> +#define _LINUX_TRACE_REMOTE_H
> +
> +#include <linux/ring_buffer.h>
> +
> +/**
> + * struct trace_remote_callbacks - Callbacks used by Tracefs to control the remote
> + *
> + * @load_trace_buffer:  Called before Tracefs accesses the trace buffer for the first
> + *			time. Must return a &trace_buffer_desc
> + *			(most likely filled with trace_remote_alloc_buffer())
> + * @unload_trace_buffer:
> + *			Called once Tracefs has no use for the trace buffer
> + *			(most likely call trace_remote_free_buffer())
> + * @enable_tracing:	Called on Tracefs tracing_on. It is expected from the
> + *			remote to allow writing.
> + * @swap_reader_page:	Called when Tracefs consumes a new page from a
> + *			ring-buffer. It is expected from the remote to isolate a
> + *			new reader-page from the @cpu ring-buffer.
> + */
> +struct trace_remote_callbacks {
> +	struct trace_buffer_desc *(*load_trace_buffer)(unsigned long size, void *priv);
> +	void	(*unload_trace_buffer)(struct trace_buffer_desc *desc, void *priv);
> +	int	(*enable_tracing)(bool enable, void *priv);
> +	int	(*swap_reader_page)(unsigned int cpu, void *priv);
> +};
> +
> +/**
> + * trace_remote_register() - Register a Tracefs remote
> + *
> + * A trace remote is an entity, outside of the kernel (most likely firmware or
> + * hypervisor) capable of writing events into a Tracefs compatible ring-buffer.
> + * The kernel would then act as a reader.
> + *
> + * The registered remote will be found under the Tracefs directory
> + * remotes/<name>.
> + *
> + * @name:	Name of the remote, used for the Tracefs remotes/ directory.
> + * @cbs:	Set of callbacks used to control the remote.
> + * @priv:	Private data, passed to each callback from @cbs.
> + * @events:	Array of events. &remote_event.name and &remote_event.id must be
> + *		filled by the caller.
> + * @nr_events:	Number of events in the @events array.
> + *
> + * Return: 0 on success, negative error code on failure.
> + */
> +int trace_remote_register(const char *name, struct trace_remote_callbacks *cbs, void *priv);

Nit, but kerneldoc goes before the functions and not before their prototypes.


> +
> +/**
> + * trace_remote_alloc_buffer() - Dynamically allocate a trace buffer
> + *
> + * Helper to dynamically allocate a set of pages (enough to cover @buffer_size)
> + * for each CPU from @cpumask and fill @desc. Most likely called from
> + * &trace_remote_callbacks.load_trace_buffer.
> + *
> + * @desc:		Uninitialized trace_buffer_desc
> + * @desc_size:		Size of the trace_buffer_desc. Must be at least equal to
> + *			trace_buffer_desc_size()
> + * @buffer_size:	Size in bytes of each per-CPU ring-buffer
> + * @cpumask:		CPUs to allocate a ring-buffer for
> + *
> + * Return: 0 on success, negative error code on failure.
> + */
> +int trace_remote_alloc_buffer(struct trace_buffer_desc *desc, size_t desc_size, size_t buffer_size,
> +			      const struct cpumask *cpumask);
> +
> +/**
> + * trace_remote_free_buffer() - Free trace buffer allocated with
> + *				trace_remote_alloc_buffer()
> + *
> + * Most likely called from &trace_remote_callbacks.unload_trace_buffer.
> + *
> + * @desc:	Descriptor of the per-CPU ring-buffers, originally filled by
> + *		trace_remote_alloc_buffer()
> + */
> +void trace_remote_free_buffer(struct trace_buffer_desc *desc);
> +
> +#endif
> diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
> index bfa2ec46e075..135edf143073 100644
> --- a/kernel/trace/Kconfig
> +++ b/kernel/trace/Kconfig
> @@ -1278,4 +1278,7 @@ config HIST_TRIGGERS_DEBUG
>  
>  source "kernel/trace/rv/Kconfig"
>  
> +config TRACE_REMOTE
> +	bool
> +
>  endif # FTRACE
> diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile
> index fc5dcc888e13..6e75d710fc25 100644
> --- a/kernel/trace/Makefile
> +++ b/kernel/trace/Makefile
> @@ -127,4 +127,5 @@ obj-$(CONFIG_FPROBE_EVENTS) += trace_fprobe.o
>  obj-$(CONFIG_TRACEPOINT_BENCHMARK) += trace_benchmark.o
>  obj-$(CONFIG_RV) += rv/
>  
> +obj-$(CONFIG_TRACE_REMOTE) += trace_remote.o
>  libftrace-y := ftrace.o
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 8bd4ec08fb36..1fcbdea992d2 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -9356,7 +9356,7 @@ static struct dentry *tracing_dentry_percpu(struct trace_array *tr, int cpu)
>  	return tr->percpu_dir;
>  }
>  
> -static struct dentry *
> +struct dentry *
>  trace_create_cpu_file(const char *name, umode_t mode, struct dentry *parent,
>  		      void *data, long cpu, const struct file_operations *fops)
>  {
> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index b6d42fe06115..fdbcd068ba38 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -680,6 +680,12 @@ struct dentry *trace_create_file(const char *name,
>  				 struct dentry *parent,
>  				 void *data,
>  				 const struct file_operations *fops);
> +struct dentry *trace_create_cpu_file(const char *name,
> +				     umode_t mode,
> +				     struct dentry *parent,
> +				     void *data,
> +				     long cpu,
> +				     const struct file_operations *fops);
>  
>  
>  /**
> diff --git a/kernel/trace/trace_remote.c b/kernel/trace/trace_remote.c
> new file mode 100644
> index 000000000000..3d80ff98fd90
> --- /dev/null
> +++ b/kernel/trace/trace_remote.c
> @@ -0,0 +1,570 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (C) 2025 - Google LLC
> + * Author: Vincent Donnefort <vdonnefort@google.com>
> + */
> +
> +#include <linux/kstrtox.h>
> +#include <linux/lockdep.h>
> +#include <linux/mutex.h>
> +#include <linux/tracefs.h>
> +#include <linux/trace_remote.h>
> +#include <linux/trace_seq.h>
> +#include <linux/types.h>
> +
> +#include "trace.h"
> +
> +#define TRACEFS_DIR		"remotes"
> +#define TRACEFS_MODE_WRITE	0640
> +#define TRACEFS_MODE_READ	0440
> +
> +struct trace_remote_iterator {
> +	struct trace_remote		*remote;
> +	struct trace_seq		seq;
> +	struct delayed_work		poll_work;
> +	unsigned long			lost_events;
> +	u64				ts;
> +	int				cpu;
> +	int				evt_cpu;
> +};
> +
> +struct trace_remote {
> +	struct trace_remote_callbacks	*cbs;
> +	void				*priv;
> +	struct trace_buffer		*trace_buffer;
> +	struct trace_buffer_desc	*trace_buffer_desc;
> +	unsigned long			trace_buffer_size;
> +	struct ring_buffer_remote	rb_remote;
> +	struct mutex			lock;
> +	unsigned int			nr_readers;
> +	unsigned int			poll_ms;
> +	bool				tracing_on;
> +};
> +
> +static bool trace_remote_loaded(struct trace_remote *remote)
> +{
> +	return remote->trace_buffer;

Nit, but since you are returning a bool, make it obvious that is what you
want:

	return !!remote->trace_buffer;



> +}
> +
> +static int trace_remote_load(struct trace_remote *remote)
> +{
> +	struct ring_buffer_remote *rb_remote = &remote->rb_remote;
> +
> +	lockdep_assert_held(&remote->lock);
> +
> +	if (trace_remote_loaded(remote))
> +		return 0;
> +
> +	remote->trace_buffer_desc = remote->cbs->load_trace_buffer(remote->trace_buffer_size,
> +								   remote->priv);
> +	if (IS_ERR(remote->trace_buffer_desc))
> +		return PTR_ERR(remote->trace_buffer_desc);

Hmm, on error, should we reset remote->trace_buffer_desc to NULL?

Just to be safe. Could just do:

	struct trace_buffer_desc *desc;

	desc = remote->cbs->load_trace_buffer(remote->trace_buffer_size, remote->priv);
	if (IS_ERR(desc))
		return PTR_ERR(desc);

	rb_remote->desc = desc;


> +
> +	rb_remote->desc = remote->trace_buffer_desc;
> +	rb_remote->swap_reader_page = remote->cbs->swap_reader_page;
> +	rb_remote->priv = remote->priv;
> +	remote->trace_buffer = ring_buffer_alloc_remote(rb_remote);
> +	if (!remote->trace_buffer) {
> +		remote->cbs->unload_trace_buffer(remote->trace_buffer_desc, remote->priv);

		remote->cbs->unload_trace_buffer(desc, remote->priv);

> +		return -ENOMEM;
> +	}

	remote->trace_buffer_desc = desc;

Only set it on success.

> +
> +	return 0;
> +}
> +
> +static void trace_remote_try_unload(struct trace_remote *remote)
> +{
> +	lockdep_assert_held(&remote->lock);
> +
> +	if (!trace_remote_loaded(remote))
> +		return;
> +
> +	/* The buffer is being read or writable */
> +	if (remote->nr_readers || remote->tracing_on)
> +		return;
> +
> +	/* The buffer has readable data */
> +	if (!ring_buffer_empty(remote->trace_buffer))
> +		return;
> +
> +	ring_buffer_free(remote->trace_buffer);
> +	remote->trace_buffer = NULL;
> +	remote->cbs->unload_trace_buffer(remote->trace_buffer_desc, remote->priv);
> +}
> +
> +static int trace_remote_enable_tracing(struct trace_remote *remote)
> +{
> +	int ret;
> +
> +	lockdep_assert_held(&remote->lock);
> +
> +	if (remote->tracing_on)
> +		return 0;
> +
> +	ret = trace_remote_load(remote);
> +	if (ret)
> +		return ret;
> +
> +	ret = remote->cbs->enable_tracing(true, remote->priv);
> +	if (ret) {
> +		trace_remote_try_unload(remote);
> +		return ret;
> +	}
> +
> +	remote->tracing_on = true;
> +
> +	return 0;
> +}
> +
> +static int trace_remote_disable_tracing(struct trace_remote *remote)
> +{
> +	int ret;
> +
> +	lockdep_assert_held(&remote->lock);
> +
> +	if (!remote->tracing_on)
> +		return 0;
> +
> +	ret = remote->cbs->enable_tracing(false, remote->priv);
> +	if (ret)
> +		return ret;
> +
> +	ring_buffer_poll_remote(remote->trace_buffer, RING_BUFFER_ALL_CPUS);
> +	remote->tracing_on = false;
> +	trace_remote_try_unload(remote);

> +
> +	return 0;
> +}
> +
> +static ssize_t
> +tracing_on_write(struct file *filp, const char __user *ubuf, size_t cnt, loff_t *ppos)
> +{
> +	struct trace_remote *remote = filp->private_data;
> +	unsigned long val;
> +	int ret;
> +
> +	ret = kstrtoul_from_user(ubuf, cnt, 10, &val);
> +	if (ret)
> +		return ret;
> +
> +	guard(mutex)(&remote->lock);
> +
> +	ret = val ? trace_remote_enable_tracing(remote) : trace_remote_disable_tracing(remote);
> +	if (ret)
> +		return ret;
> +
> +	return cnt;
> +}
> +static int tracing_on_show(struct seq_file *s, void *unused)
> +{
> +	struct trace_remote *remote = s->private;
> +
> +	seq_printf(s, "%d\n", remote->tracing_on);
> +
> +	return 0;
> +}
> +DEFINE_SHOW_STORE_ATTRIBUTE(tracing_on);
> +
> +static ssize_t buffer_size_kb_write(struct file *filp, const char __user *ubuf, size_t cnt,
> +				    loff_t *ppos)
> +{
> +	struct trace_remote *remote = filp->private_data;
> +	unsigned long val;
> +	int ret;
> +
> +	ret = kstrtoul_from_user(ubuf, cnt, 10, &val);
> +	if (ret)
> +		return ret;
> +
> +	/* KiB to Bytes */
> +	if (!val || check_shl_overflow(val, 10, &val))
> +		return -EINVAL;
> +
> +	guard(mutex)(&remote->lock);
> +
> +	remote->trace_buffer_size = val;

Should this be allowed to change when it is already loaded?

-- Steve

> +
> +	return cnt;
> +}
> +
> +static int buffer_size_kb_show(struct seq_file *s, void *unused)
> +{
> +	struct trace_remote *remote = s->private;
> +
> +	seq_printf(s, "%lu (%s)\n", remote->trace_buffer_size >> 10,
> +		   trace_remote_loaded(remote) ? "loaded" : "unloaded");
> +
> +	return 0;
> +}
> +DEFINE_SHOW_STORE_ATTRIBUTE(buffer_size_kb);
> +

[...]

> > +static ssize_t buffer_size_kb_write(struct file *filp, const char __user *ubuf, size_t cnt,
> > +				    loff_t *ppos)
> > +{
> > +	struct trace_remote *remote = filp->private_data;
> > +	unsigned long val;
> > +	int ret;
> > +
> > +	ret = kstrtoul_from_user(ubuf, cnt, 10, &val);
> > +	if (ret)
> > +		return ret;
> > +
> > +	/* KiB to Bytes */
> > +	if (!val || check_shl_overflow(val, 10, &val))
> > +		return -EINVAL;
> > +
> > +	guard(mutex)(&remote->lock);
> > +
> > +	remote->trace_buffer_size = val;
> 
> Should this be allowed to change when it is already loaded?

It would only be effective on the next unload/load. 

But now thinking more about it, this is probably not clear to a user and
returning -EBUSY when loaded might be a better solution?

> 
> -- Steve
> 
> > +
> > +	return cnt;
> > +}
> > +
> > +static int buffer_size_kb_show(struct seq_file *s, void *unused)
> > +{
> > +	struct trace_remote *remote = s->private;
> > +
> > +	seq_printf(s, "%lu (%s)\n", remote->trace_buffer_size >> 10,
> > +		   trace_remote_loaded(remote) ? "loaded" : "unloaded");
> > +
> > +	return 0;
> > +}
> > +DEFINE_SHOW_STORE_ATTRIBUTE(buffer_size_kb);
> > +
>