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

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

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.