From 946456200e4f32a5a6cb4ae851dc5a9345ec68cf Mon Sep 17 00:00:00 2001 From: Andrew Melnychenko Date: Fri, 14 May 2021 14:48:34 +0300 Subject: [PATCH] docs: Added eBPF documentation. Signed-off-by: Yuri Benditovich Signed-off-by: Andrew Melnychenko Signed-off-by: Jason Wang --- docs/devel/ebpf_rss.rst | 125 ++++++++++++++++++++++++++++++++++++++++ docs/devel/index.rst | 1 + 2 files changed, 126 insertions(+) create mode 100644 docs/devel/ebpf_rss.rst diff --git a/docs/devel/ebpf_rss.rst b/docs/devel/ebpf_rss.rst new file mode 100644 index 0000000000..e00962577a --- /dev/null +++ b/docs/devel/ebpf_rss.rst @@ -0,0 +1,125 @@ +=========================== +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()%] + + +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. diff --git a/docs/devel/index.rst b/docs/devel/index.rst index cbdbb90491..791925dcda 100644 --- a/docs/devel/index.rst +++ b/docs/devel/index.rst @@ -43,3 +43,4 @@ Contents: qom block-coroutine-wrapper multi-process + ebpf_rss