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

[Stanislav Fomichev <sdf@google.com>]

On Sat, Jun 24, 2023 at 2:02 AM Jesper Dangaard Brouer
<jbrouer@redhat.com> wrote:
>
>
>
> On 23/06/2023 19.41, Stanislav Fomichev wrote:
> > 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?
>
> Existing XDP metadata implementation also expand in a fixed/limited
> sized memory area, but communicate size per packet in this area (also
> for validation purposes).  BUT for AF_XDP we don't have room for another
> pointer or size in the AF_XDP descriptor (see struct xdp_desc).
>
>
> >
> >>> 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?
>
> I don't think so, using the meta_len (+ ifindex/netns) to communicate
> activation of TX hardware hints is too weak and not enough.  This is an
> implicit API for BPF-programmers to understand and can lead to implicit
> activation.
>
> Think about what will happen for your AF_XDP send use-case.  For
> performance reasons AF_XDP don't zero out frame memory.  Thus, meta_len
> is fixed even if not used (and can contain garbage), it can by accident
> create hard-to-debug situations.  As discussed with Magnus+Maryam
> before, we found it was practical (and faster than mem zero) to extend
> AF_XDP descriptor (see struct xdp_desc) with some flags to
> indicate/communicate this frame comes with TX metadata hints.

What is that "if not used" situation? Can the metadata itself have
is_used bit? The userspace has to initialize at least that bit.
We can definitely add that extra "has_metadata" bit to the descriptor,
but I'm trying to understand whether we can do without it.


> >>
> >> 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
> >>
> >
>