1=========================== 2eBPF RSS virtio-net support 3=========================== 4 5RSS(Receive Side Scaling) is used to distribute network packets to guest virtqueues 6by calculating packet hash. Usually every queue is processed then by a specific guest CPU core. 7 8For now there are 2 RSS implementations in qemu: 9- 'in-qemu' RSS (functions if qemu receives network packets, i.e. vhost=off) 10- eBPF RSS (can function with also with vhost=on) 11 12eBPF support (CONFIG_EBPF) is enabled by 'configure' script. 13To enable eBPF RSS support use './configure --enable-bpf'. 14 15If steering BPF is not set for kernel's TUN module, the TUN uses automatic selection 16of rx virtqueue based on lookup table built according to calculated symmetric hash 17of transmitted packets. 18If steering BPF is set for TUN the BPF code calculates the hash of packet header and 19returns the virtqueue number to place the packet to. 20 21Simplified decision formula: 22 23.. code:: C 24 25 queue_index = indirection_table[hash(<packet data>)%<indirection_table size>] 26 27 28Not for all packets, the hash can/should be calculated. 29 30Note: currently, eBPF RSS does not support hash reporting. 31 32eBPF RSS turned on by different combinations of vhost-net, vitrio-net and tap configurations: 33 34- eBPF is used: 35 36 tap,vhost=off & virtio-net-pci,rss=on,hash=off 37 38- eBPF is used: 39 40 tap,vhost=on & virtio-net-pci,rss=on,hash=off 41 42- 'in-qemu' RSS is used: 43 44 tap,vhost=off & virtio-net-pci,rss=on,hash=on 45 46- eBPF is used, hash population feature is not reported to the guest: 47 48 tap,vhost=on & virtio-net-pci,rss=on,hash=on 49 50If CONFIG_EBPF is not set then only 'in-qemu' RSS is supported. 51Also 'in-qemu' RSS, as a fallback, is used if the eBPF program failed to load or set to TUN. 52 53RSS eBPF program 54---------------- 55 56RSS program located in ebpf/rss.bpf.skeleton.h generated by bpftool. 57So the program is part of the qemu binary. 58Initially, the eBPF program was compiled by clang and source code located at tools/ebpf/rss.bpf.c. 59Prerequisites to recompile the eBPF program (regenerate ebpf/rss.bpf.skeleton.h): 60 61 llvm, clang, kernel source tree, bpftool 62 Adjust Makefile.ebpf to reflect the location of the kernel source tree 63 64 $ cd tools/ebpf 65 $ make -f Makefile.ebpf 66 67Current eBPF RSS implementation uses 'bounded loops' with 'backward jump instructions' which present in the last kernels. 68Overall eBPF RSS works on kernels 5.8+. 69 70eBPF RSS implementation 71----------------------- 72 73eBPF RSS loading functionality located in ebpf/ebpf_rss.c and ebpf/ebpf_rss.h. 74 75The ``struct EBPFRSSContext`` structure that holds 4 file descriptors: 76 77- ctx - pointer of the libbpf context. 78- program_fd - file descriptor of the eBPF RSS program. 79- map_configuration - file descriptor of the 'configuration' map. This map contains one element of 'struct EBPFRSSConfig'. This configuration determines eBPF program behavior. 80- map_toeplitz_key - file descriptor of the 'Toeplitz key' map. One element of the 40byte key prepared for the hashing algorithm. 81- map_indirections_table - 128 elements of queue indexes. 82 83``struct EBPFRSSConfig`` fields: 84 85- redirect - "boolean" value, should the hash be calculated, on false - ``default_queue`` would be used as the final decision. 86- populate_hash - for now, not used. eBPF RSS doesn't support hash reporting. 87- 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. 88- indirections_len - length of the indirections table, maximum 128. 89- 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). 90 91Functions: 92 93- ``ebpf_rss_init()`` - sets ctx to NULL, which indicates that EBPFRSSContext is not loaded. 94- ``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. 95- ``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. 96- ``ebpf_rss_unload()`` - close all file descriptors and set ctx to NULL. 97 98Simplified eBPF RSS workflow: 99 100.. code:: C 101 102 struct EBPFRSSConfig config; 103 config.redirect = 1; 104 config.hash_types = VIRTIO_NET_RSS_HASH_TYPE_UDPv4 | VIRTIO_NET_RSS_HASH_TYPE_TCPv4; 105 config.indirections_len = VIRTIO_NET_RSS_MAX_TABLE_LEN; 106 config.default_queue = 0; 107 108 uint16_t table[VIRTIO_NET_RSS_MAX_TABLE_LEN] = {...}; 109 uint8_t key[VIRTIO_NET_RSS_MAX_KEY_SIZE] = {...}; 110 111 struct EBPFRSSContext ctx; 112 ebpf_rss_init(&ctx); 113 ebpf_rss_load(&ctx); 114 ebpf_rss_set_all(&ctx, &config, table, key); 115 if (net_client->info->set_steering_ebpf != NULL) { 116 net_client->info->set_steering_ebpf(net_client, ctx->program_fd); 117 } 118 ... 119 ebpf_unload(&ctx); 120 121 122NetClientState SetSteeringEBPF() 123~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 124 125For now, ``set_steering_ebpf()`` method supported by Linux TAP NetClientState. The method requires an eBPF program file descriptor as an argument. 126