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

[Stanislav Fomichev <sdf@google.com>]

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)






> > 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).