docs: Added eBPF documentation.

Signed-off-by: Yuri Benditovich <yuri.benditovich@daynix.com>
Signed-off-by: Andrew Melnychenko <andrew@daynix.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

docs/devel/ebpf_rss.rst

+===========================
+eBPF RSS virtio-net support
+===========================
+
+RSS(Receive Side Scaling) is used to distribute network packets to guest virtqueues
+by calculating packet hash. Usually every queue is processed then by a specific guest CPU core.
+
+For now there are 2 RSS implementations in qemu:
+- 'in-qemu' RSS (functions if qemu receives network packets, i.e. vhost=off)
+- eBPF RSS (can function with also with vhost=on)
+
+eBPF support (CONFIG_EBPF) is enabled by 'configure' script.
+To enable eBPF RSS support use './configure --enable-bpf'.
+
+If steering BPF is not set for kernel's TUN module, the TUN uses automatic selection
+of rx virtqueue based on lookup table built according to calculated symmetric hash
+of transmitted packets.
+If steering BPF is set for TUN the BPF code calculates the hash of packet header and
+returns the virtqueue number to place the packet to.
+
+Simplified decision formula:
+
+.. code:: C
+
+    queue_index = indirection_table[hash(<packet data>)%<indirection_table size>]
+
+
+Not for all packets, the hash can/should be calculated.
+
+Note: currently, eBPF RSS does not support hash reporting.
+
+eBPF RSS turned on by different combinations of vhost-net, vitrio-net and tap configurations:
+
+- eBPF is used:
+
+        tap,vhost=off & virtio-net-pci,rss=on,hash=off
+
+- eBPF is used:
+
+        tap,vhost=on & virtio-net-pci,rss=on,hash=off
+
+- 'in-qemu' RSS is used:
+
+        tap,vhost=off & virtio-net-pci,rss=on,hash=on
+
+- eBPF is used, hash population feature is not reported to the guest:
+
+        tap,vhost=on & virtio-net-pci,rss=on,hash=on
+
+If CONFIG_EBPF is not set then only 'in-qemu' RSS is supported.
+Also 'in-qemu' RSS, as a fallback, is used if the eBPF program failed to load or set to TUN.
+
+RSS eBPF program
+----------------
+
+RSS program located in ebpf/rss.bpf.skeleton.h generated by bpftool.
+So the program is part of the qemu binary.
+Initially, the eBPF program was compiled by clang and source code located at tools/ebpf/rss.bpf.c.
+Prerequisites to recompile the eBPF program (regenerate ebpf/rss.bpf.skeleton.h):
+
+        llvm, clang, kernel source tree, bpftool
+        Adjust Makefile.ebpf to reflect the location of the kernel source tree
+
+        $ cd tools/ebpf
+        $ make -f Makefile.ebpf
+
+Current eBPF RSS implementation uses 'bounded loops' with 'backward jump instructions' which present in the last kernels.
+Overall eBPF RSS works on kernels 5.8+.
+
+eBPF RSS implementation
+-----------------------
+
+eBPF RSS loading functionality located in ebpf/ebpf_rss.c and ebpf/ebpf_rss.h.
+
+The `struct EBPFRSSContext` structure that holds 4 file descriptors:
+
+- ctx - pointer of the libbpf context.
+- program_fd - file descriptor of the eBPF RSS program.
+- map_configuration - file descriptor of the 'configuration' map. This map contains one element of 'struct EBPFRSSConfig'. This configuration determines eBPF program behavior.
+- map_toeplitz_key - file descriptor of the 'Toeplitz key' map. One element of the 40byte key prepared for the hashing algorithm.
+- map_indirections_table - 128 elements of queue indexes.
+
+`struct EBPFRSSConfig` fields:
+
+- redirect - "boolean" value, should the hash be calculated, on false  - `default_queue` would be used as the final decision.
+- populate_hash - for now, not used. eBPF RSS doesn't support hash reporting.
+- hash_types - binary mask of different hash types. See `VIRTIO_NET_RSS_HASH_TYPE_*` defines. If for packet hash should not be calculated - `default_queue` would be used.
+- indirections_len - length of the indirections table, maximum 128.
+- default_queue - the queue index that used for packet that shouldn't be hashed. For some packets, the hash can't be calculated(g.e ARP).
+
+Functions:
+
+- `ebpf_rss_init()` - sets ctx to NULL, which indicates that EBPFRSSContext is not loaded.
+- `ebpf_rss_load()` - creates 3 maps and loads eBPF program from the rss.bpf.skeleton.h. Returns 'true' on success. After that, program_fd can be used to set steering for TAP.
+- `ebpf_rss_set_all()` - sets values for eBPF maps. `indirections_table` length is in EBPFRSSConfig. `toeplitz_key` is VIRTIO_NET_RSS_MAX_KEY_SIZE aka 40 bytes array.
+- `ebpf_rss_unload()` - close all file descriptors and set ctx to NULL.
+
+Simplified eBPF RSS workflow:
+
+.. code:: C
+
+    struct EBPFRSSConfig config;
+    config.redirect = 1;
+    config.hash_types = VIRTIO_NET_RSS_HASH_TYPE_UDPv4 | VIRTIO_NET_RSS_HASH_TYPE_TCPv4;
+    config.indirections_len = VIRTIO_NET_RSS_MAX_TABLE_LEN;
+    config.default_queue = 0;
+
+    uint16_t table[VIRTIO_NET_RSS_MAX_TABLE_LEN] = {...};
+    uint8_t key[VIRTIO_NET_RSS_MAX_KEY_SIZE] = {...};
+
+    struct EBPFRSSContext ctx;
+    ebpf_rss_init(&ctx);
+    ebpf_rss_load(&ctx);
+    ebpf_rss_set_all(&ctx, &config, table, key);
+    if (net_client->info->set_steering_ebpf != NULL) {
+        net_client->info->set_steering_ebpf(net_client, ctx->program_fd);
+    }
+    ...
+    ebpf_unload(&ctx);
+
+
+NetClientState SetSteeringEBPF()
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+For now, `set_steering_ebpf()` method supported by Linux TAP NetClientState. The method requires an eBPF program file descriptor as an argument.

docs/devel/index.rst

@@ -43,3 +43,4 @@ Contents:
    qom
    block-coroutine-wrapper
    multi-process
+   ebpf_rss