Re: [RFC net-next 11/14] tun: run XDP program in tx path

[David Ahern <dsahern@gmail.com>]

On 12/18/19 4:48 AM, Toke Høiland-Jørgensen wrote:
> Jesper Dangaard Brouer <jbrouer@redhat.com> writes:
> 
>> On Wed, 18 Dec 2019 17:10:47 +0900
>> Prashant Bhole <prashantbhole.linux@gmail.com> wrote:
>>
>>> +static u32 tun_do_xdp_tx(struct tun_struct *tun, struct tun_file *tfile,
>>> +			 struct xdp_frame *frame)
>>> +{
>>> +	struct bpf_prog *xdp_prog;
>>> +	struct tun_page tpage;
>>> +	struct xdp_buff xdp;
>>> +	u32 act = XDP_PASS;
>>> +	int flush = 0;
>>> +
>>> +	xdp_prog = rcu_dereference(tun->xdp_tx_prog);
>>> +	if (xdp_prog) {
>>> +		xdp.data_hard_start = frame->data - frame->headroom;
>>> +		xdp.data = frame->data;
>>> +		xdp.data_end = xdp.data + frame->len;
>>> +		xdp.data_meta = xdp.data - frame->metasize;
>>
>> You have not configured xdp.rxq, thus a BPF-prog accessing this will crash.
>>
>> For an XDP TX hook, I want us to provide/give BPF-prog access to some
>> more information about e.g. the current tx-queue length, or TC-q number.
>>
>> Question to Daniel or Alexei, can we do this and still keep BPF_PROG_TYPE_XDP?
>> Or is it better to introduce a new BPF prog type (enum bpf_prog_type)
>> for XDP TX-hook ?
> 
> I think a new program type would make the most sense. If/when we
> introduce an XDP TX hook[0], it should have different semantics than the
> regular XDP hook. I view the XDP TX hook as a hook that executes as the
> very last thing before packets leave the interface. It should have
> access to different context data as you say, but also I don't think it
> makes sense to have XDP_TX and XDP_REDIRECT in an XDP_TX hook. And we
> may also want to have a "throttle" return code; or maybe that could be
> done via a helper?

XDP_TX does not make sense in the Tx path. Jason questioned whether
XDP_RX makes sense. There is not a clear use case just yet.

REDIRECT is another one that would be useful as you point out below.

A new program type would allow support for these to be added over time
and not hold up the ability to do XDP_DROP in the Tx path.

> 
> In any case, I don't think this "emulated RX hook on the other end of a
> virtual device" model that this series introduces is the right semantics
> for an XDP TX hook. I can see what you're trying to do, and for virtual
> point-to-point links I think it may make sense to emulate the RX hook of
> the "other end" on TX. However, form a UAPI perspective, I don't think
> we should be calling this a TX hook; logically, it's still an RX hook
> on the receive end.
> 
> If you guys are up for evolving this design into a "proper" TX hook (as
> outlined above an in [0]), that would be awesome, of course. But not
> sure what constraints you have on your original problem? Do you
> specifically need the "emulated RX hook for unmodified XDP programs"
> semantics, or could your problem be solved with a TX hook with different
> semantics?
> 
> -Toke
> 
> 
> [0] We've suggested this in the past, see
> https://github.com/xdp-project/xdp-project/blob/master/xdp-project.org#xdp-hook-at-tx
>