mtahhan@xxxxxxxxxx writes:
> From: Maryam Tahhan <mtahhan@xxxxxxxxxx>
>
> Add documentation for BPF_MAP_TYPE_CPUMAP including
> kernel version introduced, usage and examples.
>
> Signed-off-by: Maryam Tahhan <mtahhan@xxxxxxxxxx>
> Signed-off-by: Lorenzo Bianconi <lorenzo@xxxxxxxxxx>
> Co-developed-by: Lorenzo Bianconi <lorenzo@xxxxxxxxxx>
> ---
> Documentation/bpf/map_cpumap.rst | 140 +++++++++++++++++++++++++++++++
> 1 file changed, 140 insertions(+)
> create mode 100644 Documentation/bpf/map_cpumap.rst
>
> diff --git a/Documentation/bpf/map_cpumap.rst b/Documentation/bpf/map_cpumap.rst
> new file mode 100644
> index 000000000000..23320fb61bf7
> --- /dev/null
> +++ b/Documentation/bpf/map_cpumap.rst
> @@ -0,0 +1,140 @@
> +.. SPDX-License-Identifier: GPL-2.0-only
> +.. Copyright (C) 2022 Red Hat, Inc.
> +
> +===================
> +BPF_MAP_TYPE_CPUMAP
> +===================
> +
> +.. note::
> + - ``BPF_MAP_TYPE_CPUMAP`` was introduced in kernel version 4.15
> +
> +``BPF_MAP_TYPE_CPUMAP`` is primarily used as a backend map for the XDP BPF
> +helpers ``bpf_redirect_map()`` and ``XDP_REDIRECT`` action. This map type redirects raw
> +XDP frames to another CPU.
> +
> +A CPUMAP is a scalability and isolation mechanism that allows the steering of packets
> +to dedicated CPUs for processing. An example use-case for this map type is software
> +based Receive Side Scaling (RSS).
> +
> +The CPUMAP represents the CPUs in the system indexed as the map-key, and the
> +map-value is the config setting (per CPUMAP entry). Each CPUMAP entry has a dedicated
> +kernel thread bound to the given CPU to represent the remote CPU execution unit.
> +
> +Starting from Linux kernel version 5.9 the CPUMAP can run a second XDP program
> +on the remote CPU. This allows an XDP program to split its processing across
> +multiple CPUs. For example, a scenario where the initial CPU (that sees/receives
> +the packets) needs to do minimal packet processing and the remote CPU (to which
> +the packet is directed) can afford to spend more cycles processing the frame. The
> +initial CPU is where the XDP redirect program is executed. The remote CPU
> +receives raw``xdp_frame`` objects.
Nit - missing space between raw and ``xdp_frame`` is breaking formatting.
> +
> +Usage
> +=====
Can you add subheadings for "Kernel BPF" and "Userspace" and move
update, lookup, delete under "Userspace".
> +.. c:function::
> + long bpf_map_update_elem(struct bpf_map *map, const void *key, const void *value, u64 flags)
This function signature is for the BPF helper. If it can only be used
from userspace then this should be the libbpf function signature.
> +
> + CPU entries can be added or updated using the ``bpf_map_update_elem()``
> + helper. This helper replaces existing elements atomically. The ``value`` parameter
> + can be ``struct bpf_cpumap_val``.
I think this needs to be a stronger statement that says the value must
either be a __u32 or a struct bpf_cpumap_val.
> + .. note::
> + The maps can only be updated from user space and not from a BPF program.
Suggest moving this note to the start of the usage section.
> + .. code-block:: c
> +
> + struct bpf_cpumap_val {
> + __u32 qsize; /* queue size to remote target CPU */
> + union {
> + int fd; /* prog fd on map write */
> + __u32 id; /* prog id on map read */
> + } bpf_prog;
> + };
Should also state the valid use of flags, which I think is BPF_ANY or
BPF_EXIST due to the array semantics.
> +.. c:function::
> + void *bpf_map_lookup_elem(struct bpf_map *map, const void *key)
This needs to be the libbpf function signature.
> + CPU entries can be retrieved using the ``bpf_map_lookup_elem()``
> + helper.
> +
> +.. c:function::
> + long bpf_map_delete_elem(struct bpf_map *map, const void *key)
This needs to be the libbpf function signature.
> + CPU entries can be deleted using the ``bpf_map_delete_elem()``
> + helper. This helper will return 0 on success, or negative error in case of
> + failure.
> +
> +.. c:function::
> + long bpf_redirect_map(struct bpf_map *map, u32 key, u64 flags)
Can you put this under a "Kernel BPF" subheading.
> + Redirect the packet to the endpoint referenced by ``map`` at index ``key``.
> + For ``BPF_MAP_TYPE_CPUMAP`` this map contains references to CPUs.
> +
> + The lower two bits of *flags* are used as the return code if the map lookup
Nit - should that be ``flags``
> + fails. This is so that the return value can be one of the XDP program return
> + codes up to ``XDP_TX``, as chosen by the caller.
> +
> +Examples
> +========
> +Kernel
> +------
> +
> +The following code snippet shows how to declare a BPF_MAP_TYPE_CPUMAP called
> +cpu_map and how to redirect packets to a remote CPU using a round robin scheme.
Nit - ``BPF_MAP_TYPE_CPUMAP`` called ``cpu_map``
> +.. code-block:: c
> +
> + struct {
> + __uint(type, BPF_MAP_TYPE_CPUMAP);
> + __type(key, u32);
> + __type(value, struct bpf_cpumap_val);
> + __uint(max_entries, 12);
> + } cpu_map SEC(".maps");
> +
> + struct {
> + __uint(type, BPF_MAP_TYPE_ARRAY);
> + __type(key, u32);
> + __type(value, u32);
> + __uint(max_entries, 12);
> + } cpus_available SEC(".maps");
> +
> + struct {
> + __uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
> + __type(key, u32);
> + __type(value, u32);
> + __uint(max_entries, 1);
> + } cpus_iterator SEC(".maps");
Nit - closing brace indentation.
> + SEC("xdp")
> + int xdp_redir_cpu_round_robin(struct xdp_md *ctx)
> + {
> + u32 key = 0;
> + u32 cpu_dest = 0;
> + u32 *cpu_selected, *cpu_iterator;
> + u32 cpu_idx;
> +
> + cpu_iterator = bpf_map_lookup_elem(&cpus_iterator, &key);
> + if (!cpu_iterator)
> + return XDP_ABORTED;
> + cpu_idx = *cpu_iterator;
> +
> + *cpu_iterator += 1;
> + if (*cpu_iterator == bpf_num_possible_cpus())
> + *cpu_iterator = 0;
> +
> + cpu_selected = bpf_map_lookup_elem(&cpus_available, &cpu_idx);
> + if (!cpu_selected)
> + return XDP_ABORTED;
> + cpu_dest = *cpu_selected;
> +
> + if (cpu_dest >= bpf_num_possible_cpus())
> + return XDP_ABORTED;
> +
> + return bpf_redirect_map(&cpu_map, cpu_dest, 0);
> + }
I think the above example should use __u32 instead of u32 because it
should use UAPI definitions, but we should verify this.
> +References
> +===========
> +
> +- https://elixir.bootlin.com/linux/v6.0.1/source/kernel/bpf/cpumap.c
> +- https://developers.redhat.com/blog/2021/05/13/receive-side-scaling-rss-with-ebpf-and-cpumap#redirecting_into_a_cpumap
