[xdp-hints] Re: [RFC bpf-next v2 03/11] xsk: Support XDP_TX_METADATA_LEN

[Stanislav Fomichev <sdf@google.com>]

On Fri, Jun 23, 2023 at 3:24 AM Jesper Dangaard Brouer
<jbrouer@redhat.com> wrote:
>
>
>
> On 22/06/2023 19.55, Stanislav Fomichev wrote:
> > On Thu, Jun 22, 2023 at 2:11 AM Jesper D. Brouer <netdev@brouer.com> wrote:
> >>
> >>
> >> This needs to be reviewed by AF_XDP maintainers Magnus and Bjørn (Cc)
> >>
> >> On 21/06/2023 19.02, Stanislav Fomichev wrote:
> >>> For zerocopy mode, tx_desc->addr can point to the arbitrary offset
> >>> and carry some TX metadata in the headroom. For copy mode, there
> >>> is no way currently to populate skb metadata.
> >>>
> >>> Introduce new XDP_TX_METADATA_LEN that indicates how many bytes
> >>> to treat as metadata. Metadata bytes come prior to tx_desc address
> >>> (same as in RX case).
> >>
> >>   From looking at the code, this introduces a socket option for this TX
> >> metadata length (tx_metadata_len).
> >> This implies the same fixed TX metadata size is used for all packets.
> >> Maybe describe this in patch desc.
> >
> > I was planning to do a proper documentation page once we settle on all
> > the details (similar to the one we have for rx).
> >
> >> What is the plan for dealing with cases that doesn't populate same/full
> >> TX metadata size ?
> >
> > Do we need to support that? I was assuming that the TX layout would be
> > fixed between the userspace and BPF.
>
> I hope you don't mean fixed layout, as the whole point is adding
> flexibility and extensibility.

I do mean a fixed layout between the userspace (af_xdp) and devtx program.
At least fixed max size of the metadata. The userspace and the bpf
prog can then use this fixed space to implement some flexibility
(btf_ids, versioned structs, bitmasks, tlv, etc).
If we were to make the metalen vary per packet, we'd have to signal
its size per packet. Probably not worth it?

> > If every packet would have a different metadata length, it seems like
> > a nightmare to parse?
> >
>
> No parsing is really needed.  We can simply use BTF IDs and type cast in
> BPF-prog. Both BPF-prog and userspace have access to the local BTF ids,
> see [1] and [2].
>
> It seems we are talking slightly past each-other(?).  Let me rephrase
> and reframe the question, what is your *plan* for dealing with different
> *types* of TX metadata.  The different struct *types* will of-cause have
> different sizes, but that is okay as long as they fit into the maximum
> size set by this new socket option XDP_TX_METADATA_LEN.
> Thus, in principle I'm fine with XSK having configured a fixed headroom
> for metadata, but we need a plan for handling more than one type and
> perhaps a xsk desc indicator/flag for knowing TX metadata isn't random
> data ("leftover" since last time this mem was used).

Yeah, I think the above correctly catches my expectation here. Some
headroom is reserved via XDP_TX_METADATA_LEN and the flexibility is
offloaded to the bpf program via btf_id/tlv/etc.

Regarding leftover metadata: can we assume the userspace will take
care of setting it up?

> With this kfunc approach, then things in-principle, becomes a contract
> between the "local" TX-hook BPF-prog and AF_XDP userspace.   These two
> components can as illustrated here [1]+[2] can coordinate based on local
> BPF-prog BTF IDs.  This approach works as-is today, but patchset
> selftests examples don't use this and instead have a very static
> approach (that people will copy-paste).
>
> An unsolved problem with TX-hook is that it can also get packets from
> XDP_REDIRECT and even normal SKBs gets processed (right?).  How does the
> BPF-prog know if metadata is valid and intended to be used for e.g.
> requesting the timestamp? (imagine metadata size happen to match)

My assumption was the bpf program can do ifindex/netns filtering. Plus
maybe check that the meta_len is the one that's expected.
Will that be enough to handle XDP_REDIRECT?


> --Jesper
>
> BPF-prog API bpf_core_type_id_local:
>   - [1]
> https://github.com/xdp-project/bpf-examples/blob/master/AF_XDP-interaction/af_xdp_kern.c#L80
>
> Userspace API btf__find_by_name_kind:
>   - [2]
> https://github.com/xdp-project/bpf-examples/blob/master/AF_XDP-interaction/lib_xsk_extend.c#L185
>