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

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Magnus Karlsson <magnus.karlsson@gmail.com> writes:

> On Thu, 29 Jun 2023 at 13:30, Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>>
>> Stanislav Fomichev <sdf@google.com> writes:
>>
>> > 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.
>>
>> The reason for the flag on RX is mostly performance: there's a
>> substantial performance hit from reading the metadata area because it's
>> not cache-hot; we want to avoid that when no metadata is in use. Putting
>> the flag inside the metadata area itself doesn't work for this, because
>> then you incur the cache miss just to read the flag.
>
> Not necessarily. Let us say that the flag is 4 bytes. Increase the
> start address of the packet buffer with 4 and the flags field will be
> on the same cache line as the first 60 bytes of the packet data
> (assuming a 64 byte cache line size and the flags field is closest to
> the start of the packet data). As long as you write something in those
> first 60 bytes of packet data that cache line will be brought in and
> will likely be in the cache when you access the bits in the metadata
> field. The trick works similarly for Rx by setting the umem headroom
> accordingly.

Yeah, a trick like that was what I was alluding to with the "could" in
this bit:

>> but I see no reason it could not also occur on TX (it'll mostly
>> depend on data alignment I guess?).

right below the text you quoted ;)

> But you are correct in that dedicating a bit in the descriptor will
> make sure it is always hot, while the trick above is dependent on the
> app wanting to read or write the first cache line worth of packet
> data.

Exactly; which is why I think it's worth the flag bit :)

-Toke