On Mon, Oct 31, 2022 at 3:57 PM Martin KaFai Lau [off-list ref] wrote:

On 10/31/22 10:00 AM, Stanislav Fomichev wrote:

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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?

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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)

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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).