On 13/11/2019 20:47, Björn Töpel wrote:
> From: Björn Töpel <bjorn.topel@xxxxxxxxx>
>
> The BPF dispatcher builds on top of the BPF trampoline ideas;
> Introduce bpf_arch_text_poke() and (re-)use the BPF JIT generate
> code. The dispatcher builds a dispatch table for XDP programs, for
> retpoline avoidance. The table is a simple binary search model, so
> lookup is O(log n). Here, the dispatch table is limited to four
> entries (for laziness reason -- only 1B relative jumps :-P). If the
> dispatch table is full, it will fallback to the retpoline path.
>
> An example: A module/driver allocates a dispatcher. The dispatcher is
> shared for all netdevs. Each netdev allocate a slot in the dispatcher
> and a BPF program. The netdev then uses the dispatcher to call the
> correct program with a direct call (actually a tail-call).
>
> Signed-off-by: Björn Töpel <bjorn.topel@xxxxxxxxx>
The first-come-first-served model for dispatcher slots might mean that
a low-traffic user ends up getting priority while a higher-traffic
user is stuck with the retpoline fallback. Have you considered using
a learning mechanism, like in my dynamic call RFC [1] earlier this
year? (Though I'm sure a better learning mechanism than the one I
used there could be devised.)
> +static int bpf_dispatcher_add_prog(struct bpf_dispatcher *d,
> + struct bpf_prog *prog)
> +{
> + struct bpf_prog **entry = NULL;
> + int i, err = 0;
> +
> + if (d->num_progs == BPF_DISPATCHER_MAX)
> + return err;
> +
> + for (i = 0; i < BPF_DISPATCHER_MAX; i++) {
> + if (!entry && !d->progs[i])
> + entry = &d->progs[i];
> + if (d->progs[i] == prog)
> + return err;
> + }
> +
> + prog = bpf_prog_inc(prog);
> + if (IS_ERR(prog))
> + return err;
> +
> + *entry = prog;
> + d->num_progs++;
> + return err;
> +}
If I'm reading this function right, it always returns zero; was that
the intention, and if so why isn't it void?
-Ed
[1] https://lkml.org/lkml/2019/2/1/948
