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

[Jesper Dangaard Brouer <jbrouer@redhat.com>]

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.

>>
>> 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
>>
>