XDP offloaded mode flags set is not working with bcc - bpf

Question

I'm trying to run this tutorial XDP code that is provided in the bcc.
The code I use is this script: bcc/examples/networking/xdp/xdp_drop_count.py.
and to my understanding, XDP flag works as follows (from that question):
#define XDP_FLAGS_SKB_MODE (1U << 1)
#define XDP_FLAGS_DRV_MODE (1U << 2)
#define XDP_FLAGS_HW_MODE (1U << 3)
So, doesn't this mean that if I change the flags bit to
flags |= 1 << 3
I should be able to run this code in hardware accelerated mode (offloaded)?
I have a NIC card that supports XDP HW accelerated mode and it works fine when I just attach a simple program with only one line of code:
return XDP_PASS;
and attach it in offloaded mode by using ip link set dev interface xdpoffload etc.
So I have confirmed my NIC is capable of loading an offloaded XDP program but when I try the above, it gives me an error:
bpf: Attaching prog to enp4s0np1: Invalid argumentTraceback (most recent call last) :
File "xdp_drop_count.py", line 132, in <module>
b. attach_xdp(device, fn, flags)
File "usr/lib/python2.7/dist-packages/bcc/__init__.py", line 723, in attach_xdp % (dev, errstr))
Exception : Failed to attach BPF to device enp4s0np1: No such file or directory
Also, when I set the flags to :
flags |= 1 << 2
I am not sure if this is actually running the XDP program in driver mode.
Am I missing something?
Thank you in advance.

Answer 1

If you build bcc from sources
Since commit d147588, bcc has hardware offload support. To offload programs using bcc, you will need three things:
The XDP_FLAGS_HW_MODE bit (1U << 3) should be set in the flags passed to attach_xdp().
The name of the interface to which you want to offload the program should be given to BPF() with the device= parameter. It will allow bcc to offload the maps to the appropriate device. It is unnecessary if you don't have maps.
The interface's name should also be given to load_func, again with parameter device=, such that bcc tells the kernel where to offload the program.
Note that, with the latest bcc sources, the xdp_drop_count.py script has been updated to do all this for you when you pass the -H option:
sudo ./xdp_drop_count.py -H $ETHNAME
For older versions of bcc
Older versions of bcc do not support hardware offload. You can use bpftool or ip (>v4.16) instead, e.g.:
sudo ip link set dev $ETHNAME xdpoffload obj prog.o sec .text
sudo bpftool prog load prog.o /sys/fs/bpf/prog type xdp dev $ETHNAME

Answer 2

For a BPF program to be attached as a XDP program, it needs to be offloaded to the NIC first, when being loaded on the system.
In your case, the b.load_func() provided by bcc does not support any option for offloading programs when passing them to the kernel. So when you later call b.attach_xdp() with the XDP_FLAGS_HW_MODE, the function fails, because it cannot find any program offloaded on the NIC.
Right now there is no workaround for offloading program with bcc. As pchaigno mentioned, the function simply does not offer an option to indicate the program should be offloaded.
It should not be too difficult to add support for offloading programs to bcc though, so it should probably be available in the future (especially if pchaigno feels like adding it :p). You would still need to replace the per-CPU array by a regular array in your program, as the former is not supported for offload at this time.
Regarding the mode in which your programs run, this is something you can check with bpftool net for example.