netdev - Re: [PATCH bpf] xsk: improve documentation for AF

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Date:   Sat, 12 Oct 2019 09:47:12 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Magnus Karlsson <magnus.karlsson@...el.com>
Cc:     Björn Töpel <bjorn.topel@...el.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Network Development <netdev@...r.kernel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf] xsk: improve documentation for AF_XDP

On Thu, Oct 10, 2019 at 1:12 AM Magnus Karlsson
<magnus.karlsson@...el.com> wrote:
>
> Added sections on all the bind flags, libbpf, all the setsockopts and
> all the getsockopts. Also updated the document to reflect the latest
> features and to correct some spelling errors.
>
> Signed-off-by: Magnus Karlsson <magnus.karlsson@...el.com>

thanks for the update. Overall looks good.
Few nits below:

> +What socket will then a packet arrive on? This is decided by the XDP
> +program. Put all the sockets in the XSK_MAP and just indicate which
> +index in the array you would like to send each packet to. A simple
> +round-robin example of distributing packets is shown below:
> +
> +.. code-block:: c
> +
> +   #define KBUILD_MODNAME "af_xdp_example"

what is this for?
It's not a kernel module.

> +   #include <uapi/linux/bpf.h>

why 'uapi' ? It should use only user space headers.

> +   #include "bpf_helpers.h"
> +
> +   #define MAX_SOCKS 16
> +
> +   struct bpf_map_def SEC("maps") xsks_map = {
> +         .type = BPF_MAP_TYPE_XSKMAP,
> +         .key_size = sizeof(int),
> +         .value_size = sizeof(int),
> +         .max_entries = MAX_SOCKS,
> +   };

Could you switch to BTF defined maps?
libbpf will forever support old style as well,
but documentation should point to the latest.

> +
> +   struct bpf_map_def SEC("maps") rr_map = {
> +         .type = BPF_MAP_TYPE_PERCPU_ARRAY,
> +         .key_size = sizeof(int),
> +         .value_size = sizeof(unsigned int),
> +         .max_entries = 1,
> +   };
> +
> +   SEC("xdp_sock") int xdp_sock_prog(struct xdp_md *ctx)
> +   {
> +       int key = 0, idx;
> +       unsigned int *rr;
> +
> +       rr = bpf_map_lookup_elem(&rr_map, &key);
> +       if (!rr)
> +          return XDP_ABORTED;
> +
> +       *rr = (*rr + 1) & (MAX_SOCKS - 1);
> +       idx = *rr;
> +
> +       return bpf_redirect_map(&xsks_map, idx, 0);
> +   }
> +
> +   char _license[] SEC("license") = "GPL";

Above sample doesn't use gpl-only helpers. Why add above line?

> +.. code-block:: c
> +
> +   if (xsk_ring_prod__needs_wakeup(&my_tx_ring))
> +      sendto(xsk_socket__fd(xsk_handle), NULL, 0, MSG_DONTWAIT, NULL, 0);
> +
> +I.e., only use the syscall if the flag is set.
> +
> +We recommend that you always enable this mode as it can lead to
> +magnitudes better performance if you run the application and the
> +driver on the same core and somewhat better performance even if you
> +use different cores for the application and the kernel driver, as it
> +reduces the number of syscalls needed for the TX path.

"magnitudes better performance"? Is it really at least 20 times better?

> -Naive ring dequeue and enqueue could look like this::
> +Naive ring dequeue and enqueue could look like this:

lol. That's a good typo.