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

[Prashant Bhole <prashantbhole.linux@gmail.com>]

On 12/19/19 3:19 AM, Alexei Starovoitov wrote:
> On Wed, Dec 18, 2019 at 12:48:59PM +0100, 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?
>>
>> 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?
> 
> I agree with above.
> It looks more like existing BPF_PROG_TYPE_XDP, but attached to egress
> of veth/tap interface. I think only attachment point makes a difference.
> May be use expected_attach_type ?
> Then there will be no need to create new program type.
> BPF_PROG_TYPE_XDP will be able to access different fields depending
> on expected_attach_type. Like rx-queue length that Jesper is suggesting
> will be available only in such case and not for all BPF_PROG_TYPE_XDP progs.
> It can be reduced too. Like if there is no xdp.rxq concept for egress side
> of virtual device the access to that field can disallowed by the verifier.
> Could you also call it XDP_EGRESS instead of XDP_TX?
> I would like to reserve XDP_TX name to what Toke describes as XDP_TX.
> 

 From the discussion over this set, it makes sense to have new type of
program. As David suggested it will make a way for changes specific
to egress path.
On the other hand, XDP offload with virtio-net implementation is based
on "emulated RX hook". How about having this special behavior with
expected_attach_type?

Thanks,
Prashant