On 14/02/17 21:07, Tom Herbert wrote:
> Off the top of my head... I'd say may we might be able to have a
> minimally invasive interface with something like:
>
> XDP_RUN(hook, xdp, drv_xdp_handle_action)
>
> This replaces xdp_run and return codes are processed in the called
> functions. Its a macro so that xdp_handle_action can be inlined.
I don't see why callbacks are needed, since XDP programs (I assume)
aren't supposed to block. This XDP_RUN ends up looking a lot like
NF_HOOK, for no good reason that I can see (unlike NF hooks, we never
do things like NF_QUEUE).
> Batching could then be done in the backend XDP so that it would be
> transparent to the driver.
I also don't see how you can transparently batch and still allow the
handler to be inlined - you'd have to stash a function pointer that
you could call later when you decide to dispatch a batch of packets.
To me, the sensible interface (which makes the batching explicit to
the driver, which I think is necessary) is to have an int (or maybe
unsigned int, which is the return type of xdp_hookfn, I'm not sure
which is intended) member in struct xdp_buff.
Then the driver can call something like
XDP_RUN_ARRAY(napi, xdp_array, array_len);
which is semantically equivalent to
unsigned int i;
for (i = 0; i < array_len; i++)
xdp_array[i].ret = xdp_hook_run(napi, xdp_array + i);
except that it may run the hooks in 'row-major order'.
No callbacks needed, the driver can just loop over xdp_array reading
the .ret and applying the relevant action to each packet.
This also has the advantage that the driver knows how many packets it
might have to process in a single batch (i.e. NAPI_POLL_WEIGHT) and
can allocate the array statically, whereas an XDP hook that tried to
transparently be 'helpful' would have to guess and/or use kmalloc.
-Ed
