Re: [RFC PATCH 6/6] docs: Added eBPF documentation.

[Jason Wang <jasowang@redhat.com>]

On 2020/11/3 上午2:51, Andrew Melnychenko wrote:
> From: Andrew <andrew@daynix.com>
>
> Also, added maintainers information.
>
> Signed-off-by: Yuri Benditovich <yuri.benditovich@daynix.com>
> Signed-off-by: Andrew Melnychenko <andrew@daynix.com>
> ---
>   MAINTAINERS       |   6 +++
>   docs/ebpf.rst     |  29 +++++++++++
>   docs/ebpf_rss.rst | 129 ++++++++++++++++++++++++++++++++++++++++++++++
>   3 files changed, 164 insertions(+)
>   create mode 100644 docs/ebpf.rst
>   create mode 100644 docs/ebpf_rss.rst
>
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 2c22bbca5a..464b3f3c95 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -3111,6 +3111,12 @@ S: Maintained
>   F: hw/semihosting/
>   F: include/hw/semihosting/
>   
> +EBPF:
> +M: Andrew Melnychenko <andrew@daynix.com>
> +M: Yuri Benditovich <yuri.benditovich@daynix.com>
> +S: Maintained
> +F: ebpf/*
> +
>   Build and test automation
>   -------------------------
>   Build and test automation
> diff --git a/docs/ebpf.rst b/docs/ebpf.rst
> new file mode 100644
> index 0000000000..e45d085432
> --- /dev/null
> +++ b/docs/ebpf.rst
> @@ -0,0 +1,29 @@
> +===========================
> +eBPF qemu support
> +===========================
> +
> +eBPF support (CONFIG_EBPF) is enabled automatically by 'configure' script
> +if 'bpf' system call is available.
> +To disable eBPF support use './configure --disable-bpf'
> +
> +Basic eBPF functionality is located in ebpf/ebpf.c and ebpf/ebpf.h.
> +There are basic functions to load the eBPF program into the kernel.
> +Mostly, functions name are self-explanatory:
> +
> +- `bpf_create_map()`, `bpf_lookup_element()`, `bpf_update_element()`, `bpf_delete_element()` - manages eBPF maps. On error, a basic error message would be reported and returned -1. On success, 0 would be returned(`bpf_create_map()` returns map's file descriptor).
> +- `bpf_prog_load()` - load the program. The program has to have proper map file descriptors if there are used. On error - the log eBPF would be reported. On success, the program file descriptor returned.
> +- `bpf_fixup_mapfd()` - would place map file descriptor into the program according to 'relocate array' of 'struct fixup_mapfd_t'. The function would return how many instructions were 'fixed' aka how many relocations was occurred.
> +
> +Simplified workflow would look like this:
> +
> +.. code:: C
> +
> +    int map1 = bpf_create_map(...);
> +    int map2 = bpf_create_map(...);
> +
> +    bpf_fixup_mapfd(<fixup table>, ARRAY_SIZE(<fixup table>), <instructions pointer>, ARRAY_SIZE(<instructions pointer>), <map1 name>, map1);
> +    bpf_fixup_mapfd(<fixup table>, ARRAY_SIZE(<fixup table>), <instructions pointer>, ARRAY_SIZE(<instructions pointer>), <map2 name>, map2);
> +
> +    int prog = bpf_prog_load(<program type>, <instructions pointer>, ARRAY_SIZE(<instructions pointer>), "GPL");
> +
> +See the bpf(2) for details.
> diff --git a/docs/ebpf_rss.rst b/docs/ebpf_rss.rst
> new file mode 100644
> index 0000000000..96fee391b8
> --- /dev/null
> +++ b/docs/ebpf_rss.rst
> @@ -0,0 +1,129 @@
> +===========================
> +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:
> +- 'software' RSS (functions if qemu receives network packets, i.e. vhost=off)
> +- eBPF RSS (can function with also with vhost=on)
> +
> +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
> +
> +- 'software' 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 'software' RSS is supported.
> +Also 'software' 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/tun_rss_steering.h as an array of 'struct bpf_insn'.
> +So the program is part of the qemu binary.
> +Initially, the eBPF program was compiled by clang and source code located at ebpf/rss.bpf.c.
> +Prerequisites to recompile the eBPF program (regenerate ebpf/tun_rss_steering.h):
> +
> +        llvm, clang, kernel source tree, python3 + (pip3 pyelftools)
> +        Adjust 'linuxhdrs' in Makefile.ebpf to reflect the location of the kernel source tree
> +
> +        $ cd ebpf
> +        $ make -f Makefile.ebpf
> +
> +Note the python script for convertation from eBPF ELF object to '.h' file - Ebpf_to_C.py:
> +
> +        $ python EbpfElf_to_C.py rss.bpf.o tun_rss_steering
> +
> +The first argument of the script is ELF object, second - section name where the eBPF program located.
> +The script would generate <section name>.h file with eBPF instructions and 'relocate array'.
> +'relocate array' is an array of 'struct fixup_mapfd_t' with the name of the eBPF map and instruction offset where the file descriptor of the map should be placed.
> +


Do we still need this if we decide to use llvm/clang toolchain? (I guess 
not)

Thanks


> +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:
> +
> +- 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 program_fd to -1, which indicates that EBPFRSSContext is not loaded.
> +- `ebpf_rss_load()` - creates 3 maps and loads eBPF program from tun_rss_steering.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 program_fd to -1.
> +
> +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.