f0d7b970ac
In rST markup, single backticks `like this` represent "interpreted text", which can be handled as a bunch of different things if tagged with a specific "role": https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html#interpreted-text (the most common one for us is "reference to a URL, which gets hyperlinked"). The default "role" if none is specified is "title_reference", intended for references to book or article titles, and it renders into the HTML as <cite>...</cite> (usually comes out as italics). To format a literal (generally rendered as fixed-width font), double-backticks are required. ebpf_rss.rst gets this wrong in a few places; correct them. Signed-off-by: Peter Maydell <peter.maydell@linaro.org> Reviewed-by: Richard Henderson <richard.henderson@linaro.org> Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com> Message-id: 20210726142338.31872-4-peter.maydell@linaro.org
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===========================
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eBPF RSS virtio-net support
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===========================
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RSS(Receive Side Scaling) is used to distribute network packets to guest virtqueues
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by calculating packet hash. Usually every queue is processed then by a specific guest CPU core.
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For now there are 2 RSS implementations in qemu:
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- 'in-qemu' RSS (functions if qemu receives network packets, i.e. vhost=off)
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- eBPF RSS (can function with also with vhost=on)
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eBPF support (CONFIG_EBPF) is enabled by 'configure' script.
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To enable eBPF RSS support use './configure --enable-bpf'.
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If steering BPF is not set for kernel's TUN module, the TUN uses automatic selection
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of rx virtqueue based on lookup table built according to calculated symmetric hash
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of transmitted packets.
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If steering BPF is set for TUN the BPF code calculates the hash of packet header and
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returns the virtqueue number to place the packet to.
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Simplified decision formula:
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.. code:: C
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queue_index = indirection_table[hash(<packet data>)%<indirection_table size>]
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Not for all packets, the hash can/should be calculated.
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Note: currently, eBPF RSS does not support hash reporting.
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eBPF RSS turned on by different combinations of vhost-net, vitrio-net and tap configurations:
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- eBPF is used:
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tap,vhost=off & virtio-net-pci,rss=on,hash=off
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- eBPF is used:
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tap,vhost=on & virtio-net-pci,rss=on,hash=off
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- 'in-qemu' RSS is used:
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tap,vhost=off & virtio-net-pci,rss=on,hash=on
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- eBPF is used, hash population feature is not reported to the guest:
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tap,vhost=on & virtio-net-pci,rss=on,hash=on
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If CONFIG_EBPF is not set then only 'in-qemu' RSS is supported.
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Also 'in-qemu' RSS, as a fallback, is used if the eBPF program failed to load or set to TUN.
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RSS eBPF program
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----------------
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RSS program located in ebpf/rss.bpf.skeleton.h generated by bpftool.
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So the program is part of the qemu binary.
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Initially, the eBPF program was compiled by clang and source code located at tools/ebpf/rss.bpf.c.
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Prerequisites to recompile the eBPF program (regenerate ebpf/rss.bpf.skeleton.h):
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llvm, clang, kernel source tree, bpftool
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Adjust Makefile.ebpf to reflect the location of the kernel source tree
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$ cd tools/ebpf
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$ make -f Makefile.ebpf
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Current eBPF RSS implementation uses 'bounded loops' with 'backward jump instructions' which present in the last kernels.
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Overall eBPF RSS works on kernels 5.8+.
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eBPF RSS implementation
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-----------------------
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eBPF RSS loading functionality located in ebpf/ebpf_rss.c and ebpf/ebpf_rss.h.
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The ``struct EBPFRSSContext`` structure that holds 4 file descriptors:
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- ctx - pointer of the libbpf context.
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- program_fd - file descriptor of the eBPF RSS program.
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- map_configuration - file descriptor of the 'configuration' map. This map contains one element of 'struct EBPFRSSConfig'. This configuration determines eBPF program behavior.
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- map_toeplitz_key - file descriptor of the 'Toeplitz key' map. One element of the 40byte key prepared for the hashing algorithm.
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- map_indirections_table - 128 elements of queue indexes.
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``struct EBPFRSSConfig`` fields:
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- redirect - "boolean" value, should the hash be calculated, on false - ``default_queue`` would be used as the final decision.
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- populate_hash - for now, not used. eBPF RSS doesn't support hash reporting.
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- 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.
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- indirections_len - length of the indirections table, maximum 128.
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- 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).
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Functions:
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- ``ebpf_rss_init()`` - sets ctx to NULL, which indicates that EBPFRSSContext is not loaded.
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- ``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.
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- ``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.
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- ``ebpf_rss_unload()`` - close all file descriptors and set ctx to NULL.
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Simplified eBPF RSS workflow:
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.. code:: C
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struct EBPFRSSConfig config;
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config.redirect = 1;
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config.hash_types = VIRTIO_NET_RSS_HASH_TYPE_UDPv4 | VIRTIO_NET_RSS_HASH_TYPE_TCPv4;
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config.indirections_len = VIRTIO_NET_RSS_MAX_TABLE_LEN;
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config.default_queue = 0;
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uint16_t table[VIRTIO_NET_RSS_MAX_TABLE_LEN] = {...};
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uint8_t key[VIRTIO_NET_RSS_MAX_KEY_SIZE] = {...};
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struct EBPFRSSContext ctx;
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ebpf_rss_init(&ctx);
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ebpf_rss_load(&ctx);
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ebpf_rss_set_all(&ctx, &config, table, key);
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if (net_client->info->set_steering_ebpf != NULL) {
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net_client->info->set_steering_ebpf(net_client, ctx->program_fd);
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}
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...
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ebpf_unload(&ctx);
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NetClientState SetSteeringEBPF()
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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For now, ``set_steering_ebpf()`` method supported by Linux TAP NetClientState. The method requires an eBPF program file descriptor as an argument.
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