[xdp-hints] Re: [RFC bpf-next 0/5] xdp: hints via kfuncs

[Martin KaFai Lau <martin.lau@linux.dev>]

On 10/31/22 6:59 PM, Stanislav Fomichev wrote:
> On Mon, Oct 31, 2022 at 3:57 PM Martin KaFai Lau <martin.lau@linux.dev> wrote:
>>
>> On 10/31/22 10:00 AM, Stanislav Fomichev wrote:
>>>> 2. AF_XDP programs won't be able to access the metadata without using a
>>>> custom XDP program that calls the kfuncs and puts the data into the
>>>> metadata area. We could solve this with some code in libxdp, though; if
>>>> this code can be made generic enough (so it just dumps the available
>>>> metadata functions from the running kernel at load time), it may be
>>>> possible to make it generic enough that it will be forward-compatible
>>>> with new versions of the kernel that add new fields, which should
>>>> alleviate Florian's concern about keeping things in sync.
>>>
>>> Good point. I had to convert to a custom program to use the kfuncs :-(
>>> But your suggestion sounds good; maybe libxdp can accept some extra
>>> info about at which offset the user would like to place the metadata
>>> and the library can generate the required bytecode?
>>>
>>>> 3. It will make it harder to consume the metadata when building SKBs. I
>>>> think the CPUMAP and veth use cases are also quite important, and that
>>>> we want metadata to be available for building SKBs in this path. Maybe
>>>> this can be resolved by having a convenient kfunc for this that can be
>>>> used for programs doing such redirects. E.g., you could just call
>>>> xdp_copy_metadata_for_skb() before doing the bpf_redirect, and that
>>>> would recursively expand into all the kfunc calls needed to extract the
>>>> metadata supported by the SKB path?
>>>
>>> So this xdp_copy_metadata_for_skb will create a metadata layout that
>>
>> Can the xdp_copy_metadata_for_skb be written as a bpf prog itself?
>> Not sure where is the best point to specify this prog though.  Somehow during
>> bpf_xdp_redirect_map?
>> or this prog belongs to the target cpumap and the xdp prog redirecting to this
>> cpumap has to write the meta layout in a way that the cpumap is expecting?
> 
> We're probably interested in triggering it from the places where xdp
> frames can eventually be converted into skbs?
> So for plain 'return XDP_PASS' and things like bpf_redirect/etc? (IOW,
> anything that's not XDP_DROP / AF_XDP redirect).
> We can probably make it magically work, and can generate
> kernel-digestible metadata whenever data == data_meta, but the
> question - should we?
> (need to make sure we won't regress any existing cases that are not
> relying on the metadata)

Instead of having some kernel-digestible meta data, how about calling another 
bpf prog to initialize the skb fields from the meta area after 
__xdp_build_skb_from_frame() in the cpumap, so 
run_xdp_set_skb_fileds_from_metadata() may be a better name.

The xdp_prog@rx sets the meta data and then redirect.  If the xdp_prog@rx can 
also specify a xdp prog to initialize the skb fields from the meta area, then 
there is no need to have a kfunc to enforce a kernel-digestible layout.  Not 
sure what is a good way to specify this xdp_prog though...

>>> the kernel will be able to understand when converting back to skb?
>>> IIUC, the xdp program will look something like the following:
>>>
>>> if (xdp packet is to be consumed by af_xdp) {
>>>     // do a bunch of bpf_xdp_metadata_<metadata> calls and assemble your
>>> own metadata layout
>>>     return bpf_redirect_map(xsk, ...);
>>> } else {
>>>     // if the packet is to be consumed by the kernel
>>>     xdp_copy_metadata_for_skb(ctx);
>>>     return bpf_redirect(...);
>>> }
>>>
>>> Sounds like a great suggestion! xdp_copy_metadata_for_skb can maybe
>>> put some magic number in the first byte(s) of the metadata so the
>>> kernel can check whether xdp_copy_metadata_for_skb has been called
>>> previously (or maybe xdp_frame can carry this extra signal, idk).