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

[Magnus Karlsson <magnus.karlsson@gmail.com>]

On Wed, 28 Jun 2023 at 20:49, Stanislav Fomichev <sdf@google.com> wrote:
>
> On Wed, Jun 28, 2023 at 1:09 AM Magnus Karlsson
> <magnus.karlsson@gmail.com> wrote:
> >
> > On Mon, 26 Jun 2023 at 19:06, Stanislav Fomichev <sdf@google.com> wrote:
> > >
> > > 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.
> >
> > To me, this "has_metadata" bit in the descriptor is just an
> > optimization. If it is 0, then there is no need to go and check the
> > metadata field and you save some performance. Regardless of this bit,
> > you need some way to say "is_used" for each metadata entry (at least
> > when the number of metadata entries is >1). Three options come to mind
> > each with their pros and cons.
> >
> > #1: Let each metadata entry have an invalid state. Not possible for
> > every metadata and requires the user/kernel to go scan through every
> > entry for every packet.
> >
> > #2: Have a field of bits at the start of the metadata section (closest
> > to packet data) that signifies if a metadata entry is valid or not. If
> > there are N metadata entries in the metadata area, then N bits in this
> > field would be used to signify if the corresponding metadata is used
> > or not. Only requires the user/kernel to scan the valid entries plus
> > one access for the "is_used" bits.
> >
> > #3: Have N bits in the AF_XDP descriptor options field instead of the
> > N bits in the metadata area of #2. Faster but would consume many
> > precious bits in the fixed descriptor and cap the number of metadata
> > entries possible at around 8. E.g., 8 for Rx, 8 for Tx, 1 for the
> > multi-buffer work, and 15 for some future use. Depends on how daring
> > we are.
> >
> > The "has_metadata" bit suggestion can be combined with 1 or 2.
> > Approach 3 is just a fine grained extension of the idea itself.
> >
> > IMO, the best approach unfortunately depends on the metadata itself.
> > If it is rarely valid, you want something like the "has_metadata" bit.
> > If it is nearly always valid and used, approach #1 (if possible for
> > the metadata) should be the fastest. The decision also depends on the
> > number of metadata entries you have per packet. Sorry that I do not
> > have a good answer. My feeling is that we need something like #1 or
> > #2, or maybe both, then if needed we can add the "has_metadata" bit or
> > bits (#3) optimization. Can we do this encoding and choice (#1, #2, or
> > a combo) in the eBPF program itself? Would provide us with the
> > flexibility, if possible.
>
> Here is my take on it, lmk if I'm missing something:
>
> af_xdp users call this new setsockopt(XDP_TX_METADATA_LEN) when they
> plan to use metadata on tx.
> This essentially requires allocating a fixed headroom to carry the metadata.
> af_xdp machinery exports this fixed len into the bpf programs somehow
> (devtx_frame.meta_len in this series).
> Then it's up to the userspace and bpf program to agree on the layout.
> If not every packet is expected to carry the metadata, there might be
> some bitmask in the metadata area to indicate that.
>
> Iow, the metadata isn't interpreted by the kernel. It's up to the prog
> to interpret it and call appropriate kfunc to enable some offload.

Sounds good. This flexibility is needed.

> Jesper raises a valid point with "what about redirected packets?". But
> I'm not sure we need to care? Presumably the programs that do
> xdp_redirect will have to conform to the same metadata layout?