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+.. SPDX-License-Identifier: GPL-2.0
+
+.. _cpumasks-header-label:
+
+==================
+BPF cpumask kfuncs
+==================
+
+1. Introduction
+===============
+
+``struct cpumask`` is a bitmap data structure in the kernel whose indices
+reflect the CPUs on the system. Commonly, cpumasks are used to track which CPUs
+a task is affinitized to, but they can also be used to e.g. track which cores
+are associated with a scheduling domain, which cores on a machine are idle,
+etc.
+
+BPF provides programs with a set of :ref:`kfuncs-header-label` that can be
+used to allocate, mutate, query, and free cpumasks.
+
+2. BPF cpumask objects
+======================
+
+There are two different types of cpumasks that can be used by BPF programs.
+
+2.1 ``struct bpf_cpumask *``
+----------------------------
+
+``struct bpf_cpumask *`` is a cpumask that is allocated by BPF, on behalf of a
+BPF program, and whose lifecycle is entirely controlled by BPF. These cpumasks
+are RCU-protected, can be mutated, can be used as kptrs, and can be safely cast
+to a ``struct cpumask *``.
+
+2.1.1 ``struct bpf_cpumask *`` lifecycle
+----------------------------------------
+
+A ``struct bpf_cpumask *`` is allocated, acquired, and released, using the
+following functions:
+
+.. kernel-doc:: kernel/bpf/cpumask.c
+ :identifiers: bpf_cpumask_create
+
+.. kernel-doc:: kernel/bpf/cpumask.c
+ :identifiers: bpf_cpumask_acquire
+
+.. kernel-doc:: kernel/bpf/cpumask.c
+ :identifiers: bpf_cpumask_release
+
+For example:
+
+.. code-block:: c
+
+ struct cpumask_map_value {
+ struct bpf_cpumask __kptr * cpumask;
+ };
+
+ struct array_map {
+ __uint(type, BPF_MAP_TYPE_ARRAY);
+ __type(key, int);
+ __type(value, struct cpumask_map_value);
+ __uint(max_entries, 65536);
+ } cpumask_map SEC(".maps");
+
+ static int cpumask_map_insert(struct bpf_cpumask *mask, u32 pid)
+ {
+ struct cpumask_map_value local, *v;
+ long status;
+ struct bpf_cpumask *old;
+ u32 key = pid;
+
+ local.cpumask = NULL;
+ status = bpf_map_update_elem(&cpumask_map, &key, &local, 0);
+ if (status) {
+ bpf_cpumask_release(mask);
+ return status;
+ }
+
+ v = bpf_map_lookup_elem(&cpumask_map, &key);
+ if (!v) {
+ bpf_cpumask_release(mask);
+ return -ENOENT;
+ }
+
+ old = bpf_kptr_xchg(&v->cpumask, mask);
+ if (old)
+ bpf_cpumask_release(old);
+
+ return 0;
+ }
+
+ /**
+ * A sample tracepoint showing how a task's cpumask can be queried and
+ * recorded as a kptr.
+ */
+ SEC("tp_btf/task_newtask")
+ int BPF_PROG(record_task_cpumask, struct task_struct *task, u64 clone_flags)
+ {
+ struct bpf_cpumask *cpumask;
+ int ret;
+
+ cpumask = bpf_cpumask_create();
+ if (!cpumask)
+ return -ENOMEM;
+
+ if (!bpf_cpumask_full(task->cpus_ptr))
+ bpf_printk("task %s has CPU affinity", task->comm);
+
+ bpf_cpumask_copy(cpumask, task->cpus_ptr);
+ return cpumask_map_insert(cpumask, task->pid);
+ }
+
+----
+
+2.1.1 ``struct bpf_cpumask *`` as kptrs
+---------------------------------------
+
+As mentioned and illustrated above, these ``struct bpf_cpumask *`` objects can
+also be stored in a map and used as kptrs. If a ``struct bpf_cpumask *`` is in
+a map, the reference can be removed from the map with bpf_kptr_xchg(), or
+opportunistically acquired using RCU:
+
+.. code-block:: c
+
+ /* struct containing the struct bpf_cpumask kptr which is stored in the map. */
+ struct cpumasks_kfunc_map_value {
+ struct bpf_cpumask __kptr * bpf_cpumask;
+ };
+
+ /* The map containing struct cpumasks_kfunc_map_value entries. */
+ struct {
+ __uint(type, BPF_MAP_TYPE_ARRAY);
+ __type(key, int);
+ __type(value, struct cpumasks_kfunc_map_value);
+ __uint(max_entries, 1);
+ } cpumasks_kfunc_map SEC(".maps");
+
+ /* ... */
+
+ /**
+ * A simple example tracepoint program showing how a
+ * struct bpf_cpumask * kptr that is stored in a map can
+ * be passed to kfuncs using RCU protection.
+ */
+ SEC("tp_btf/cgroup_mkdir")
+ int BPF_PROG(cgrp_ancestor_example, struct cgroup *cgrp, const char *path)
+ {
+ struct bpf_cpumask *kptr;
+ struct cpumasks_kfunc_map_value *v;
+ u32 key = 0;
+
+ /* Assume a bpf_cpumask * kptr was previously stored in the map. */
+ v = bpf_map_lookup_elem(&cpumasks_kfunc_map, &key);
+ if (!v)
+ return -ENOENT;
+
+ bpf_rcu_read_lock();
+ /* Acquire a reference to the bpf_cpumask * kptr that's already stored in the map. */
+ kptr = v->cpumask;
+ if (!kptr) {
+ /* If no bpf_cpumask was present in the map, it's because
+ * we're racing with another CPU that removed it with
+ * bpf_kptr_xchg() between the bpf_map_lookup_elem()
+ * above, and our load of the pointer from the map.
+ */
+ bpf_rcu_read_unlock();
+ return -EBUSY;
+ }
+
+ bpf_cpumask_setall(kptr);
+ bpf_rcu_read_unlock();
+
+ return 0;
+ }
+
+----
+
+2.2 ``struct cpumask``
+----------------------
+
+``struct cpumask`` is the object that actually contains the cpumask bitmap
+being queried, mutated, etc. A ``struct bpf_cpumask`` wraps a ``struct
+cpumask``, which is why it's safe to cast it as such (note however that it is
+**not** safe to cast a ``struct cpumask *`` to a ``struct bpf_cpumask *``, and
+the verifier will reject any program that tries to do so).
+
+As we'll see below, any kfunc that mutates its cpumask argument will take a
+``struct bpf_cpumask *`` as that argument. Any argument that simply queries the
+cpumask will instead take a ``struct cpumask *``.
+
+3. cpumask kfuncs
+=================
+
+Above, we described the kfuncs that can be used to allocate, acquire, release,
+etc a ``struct bpf_cpumask *``. This section of the document will describe the
+kfuncs for mutating and querying cpumasks.
+
+3.1 Mutating cpumasks
+---------------------
+
+Some cpumask kfuncs are "read-only" in that they don't mutate any of their
+arguments, whereas others mutate at least one argument (which means that the
+argument must be a ``struct bpf_cpumask *``, as described above).
+
+This section will describe all of the cpumask kfuncs which mutate at least one
+argument. :ref:`cpumasks-querying-label` below describes the read-only kfuncs.
+
+3.1.1 Setting and clearing CPUs
+-------------------------------
+
+bpf_cpumask_set_cpu() and bpf_cpumask_clear_cpu() can be used to set and clear
+a CPU in a ``struct bpf_cpumask`` respectively:
+
+.. kernel-doc:: kernel/bpf/cpumask.c
+ :identifiers: bpf_cpumask_set_cpu bpf_cpumask_clear_cpu
+
+These kfuncs are pretty straightforward, and can be used, for example, as
+follows:
+
+.. code-block:: c
+
+ /**
+ * A sample tracepoint showing how a cpumask can be queried.
+ */
+ SEC("tp_btf/task_newtask")
+ int BPF_PROG(test_set_clear_cpu, struct task_struct *task, u64 clone_flags)
+ {
+ struct bpf_cpumask *cpumask;
+
+ cpumask = bpf_cpumask_create();
+ if (!cpumask)
+ return -ENOMEM;
+
+ bpf_cpumask_set_cpu(0, cpumask);
+ if (!bpf_cpumask_test_cpu(0, cast(cpumask)))
+ /* Should never happen. */
+ goto release_exit;
+
+ bpf_cpumask_clear_cpu(0, cpumask);
+ if (bpf_cpumask_test_cpu(0, cast(cpumask)))
+ /* Should never happen. */
+ goto release_exit;
+
+ /* struct cpumask * pointers such as task->cpus_ptr can also be queried. */
+ if (bpf_cpumask_test_cpu(0, task->cpus_ptr))
+ bpf_printk("task %s can use CPU %d", task->comm, 0);
+
+ release_exit:
+ bpf_cpumask_release(cpumask);
+ return 0;
+ }
+
+----
+
+bpf_cpumask_test_and_set_cpu() and bpf_cpumask_test_and_clear_cpu() are
+complementary kfuncs that allow callers to atomically test and set (or clear)
+CPUs:
+
+.. kernel-doc:: kernel/bpf/cpumask.c
+ :identifiers: bpf_cpumask_test_and_set_cpu bpf_cpumask_test_and_clear_cpu
+
+----
+
+We can also set and clear entire ``struct bpf_cpumask *`` objects in one
+operation using bpf_cpumask_setall() and bpf_cpumask_clear():
+
+.. kernel-doc:: kernel/bpf/cpumask.c
+ :identifiers: bpf_cpumask_setall bpf_cpumask_clear
+
+3.1.2 Operations between cpumasks
+---------------------------------
+
+In addition to setting and clearing individual CPUs in a single cpumask,
+callers can also perform bitwise operations between multiple cpumasks using
+bpf_cpumask_and(), bpf_cpumask_or(), and bpf_cpumask_xor():
+
+.. kernel-doc:: kernel/bpf/cpumask.c
+ :identifiers: bpf_cpumask_and bpf_cpumask_or bpf_cpumask_xor
+
+The following is an example of how they may be used. Note that some of the
+kfuncs shown in this example will be covered in more detail below.
+
+.. code-block:: c
+
+ /**
+ * A sample tracepoint showing how a cpumask can be mutated using
+ bitwise operators (and queried).
+ */
+ SEC("tp_btf/task_newtask")
+ int BPF_PROG(test_and_or_xor, struct task_struct *task, u64 clone_flags)
+ {
+ struct bpf_cpumask *mask1, *mask2, *dst1, *dst2;
+
+ mask1 = bpf_cpumask_create();
+ if (!mask1)
+ return -ENOMEM;
+
+ mask2 = bpf_cpumask_create();
+ if (!mask2) {
+ bpf_cpumask_release(mask1);
+ return -ENOMEM;
+ }
+
+ // ...Safely create the other two masks... */
+
+ bpf_cpumask_set_cpu(0, mask1);
+ bpf_cpumask_set_cpu(1, mask2);
+ bpf_cpumask_and(dst1, (const struct cpumask *)mask1, (const struct cpumask *)mask2);
+ if (!bpf_cpumask_empty((const struct cpumask *)dst1))
+ /* Should never happen. */
+ goto release_exit;
+
+ bpf_cpumask_or(dst1, (const struct cpumask *)mask1, (const struct cpumask *)mask2);
+ if (!bpf_cpumask_test_cpu(0, (const struct cpumask *)dst1))
+ /* Should never happen. */
+ goto release_exit;
+
+ if (!bpf_cpumask_test_cpu(1, (const struct cpumask *)dst1))
+ /* Should never happen. */
+ goto release_exit;
+
+ bpf_cpumask_xor(dst2, (const struct cpumask *)mask1, (const struct cpumask *)mask2);
+ if (!bpf_cpumask_equal((const struct cpumask *)dst1,
+ (const struct cpumask *)dst2))
+ /* Should never happen. */
+ goto release_exit;
+
+ release_exit:
+ bpf_cpumask_release(mask1);
+ bpf_cpumask_release(mask2);
+ bpf_cpumask_release(dst1);
+ bpf_cpumask_release(dst2);
+ return 0;
+ }
+
+----
+
+The contents of an entire cpumask may be copied to another using
+bpf_cpumask_copy():
+
+.. kernel-doc:: kernel/bpf/cpumask.c
+ :identifiers: bpf_cpumask_copy
+
+----
+
+.. _cpumasks-querying-label:
+
+3.2 Querying cpumasks
+---------------------
+
+In addition to the above kfuncs, there is also a set of read-only kfuncs that
+can be used to query the contents of cpumasks.
+
+.. kernel-doc:: kernel/bpf/cpumask.c
+ :identifiers: bpf_cpumask_first bpf_cpumask_first_zero bpf_cpumask_first_and
+ bpf_cpumask_test_cpu
+
+.. kernel-doc:: kernel/bpf/cpumask.c
+ :identifiers: bpf_cpumask_equal bpf_cpumask_intersects bpf_cpumask_subset
+ bpf_cpumask_empty bpf_cpumask_full
+
+.. kernel-doc:: kernel/bpf/cpumask.c
+ :identifiers: bpf_cpumask_any_distribute bpf_cpumask_any_and_distribute
+
+----
+
+Some example usages of these querying kfuncs were shown above. We will not
+replicate those examples here. Note, however, that all of the aforementioned
+kfuncs are tested in `tools/testing/selftests/bpf/progs/cpumask_success.c`_, so
+please take a look there if you're looking for more examples of how they can be
+used.
+
+.. _tools/testing/selftests/bpf/progs/cpumask_success.c:
+ https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/testing/selftests/bpf/progs/cpumask_success.c
+
+
+4. Adding BPF cpumask kfuncs
+============================
+
+The set of supported BPF cpumask kfuncs are not (yet) a 1-1 match with the
+cpumask operations in include/linux/cpumask.h. Any of those cpumask operations
+could easily be encapsulated in a new kfunc if and when required. If you'd like
+to support a new cpumask operation, please feel free to submit a patch. If you
+do add a new cpumask kfunc, please document it here, and add any relevant
+selftest testcases to the cpumask selftest suite.