Re: XDP-hints: Howto support multiple BTF types per packet basis?

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

On 09/09/2021 20.19, Andrii Nakryiko wrote:
> Depending on what IDs we are talking about (sorry, I don't follow this
> thread very closely, so if you are curious about some aspects of BTF
> or libbpf APIs, it would be good to have a specific questions with
> some context). BTF as kernel object has it's own ID allocated through
> idr, so yes, they are unique. so vmlinux BTF object will have it's own
> ID, while each module's BTF will have it's own.
> 
> But if we are talking about BTF type IDs, that's entirely different
> thing. BTF type IDs start from 1 (0 is reserved for special 'VOID'
> type) all the way to number of types in vmlinux BTF. Then each module
> extends vmlinx BTF starting at N + 1 and going to N + M, where N is
> number of BTF types in vmlinux BTF and M is number of added types in
> module BTF.
> 
> So in that regard each module has BTF type IDs that are overlapping
> with other modules, which is why for unique fetching of BTF types from
> modules you also need BTF object FD or ID of a module BTF, and then
> BTF type ID within that module. But as I said, I didn't follow along
> closely, so not sure if I'm answering the right question, sorry.

Thanks for answering.  This N vmlinux IDs + M module IDs was important 
to know, thanks for correcting my understanding on this, as this does 
affect our ideas for using BTF for XDP-hints.

This "just" means that the BTF ID will be per driver.  I think we can 
still make this work, as the AF_XDP userspace program will already need 
to bind to a device.  Thus, we can still send a simple btf_id in 
metadata, and AF_XDP prog will just have device-map with expected 
btf_id's from this device (to validate if it knows howto decode contents).
It is slightly more annoying for my xdp_frame + cpumap use-case, as it 
can get XDP_REDIRECT'ed frames from many net_devices, but we do have 
xdp_frame->dev_rx (net_device) avail, so I can resolve this.

--Jesper

Finding some random BTF ID in two module and notice they point to 
different types.

  # bpftool btf dump file /sys/kernel/btf/ixgbe | grep 95905
  [95905] FUNC 'ixgbe_set_rx_mode' type_id=95829 linkage=static

  # bpftool btf dump file /sys/kernel/btf/igc | grep 95905
  [95905] FUNC 'igc_ethtool_get_link_ksettings' type_id=95904 linkage=static