I want to implement XDP_SHARED_UMEM: https://www.kernel.org/doc/html/latest/networking/af_xdp.html#xdp-shared-umem-bind-flag
The libbpf library function xsk_socket__create (https://github.com/libbpf/libbpf/blob/master/src/xsk.c) checks the xsk_umem->refcount value. In case it is greater than 1, the XDP_SHARED_UMEM option of a struct sockaddr_xdp is set.
So as far as I understand correctly, I "just" need to pass the original umem struct of the socket I want to share the umem with and the rest is done by libbpf.
The way I tried to do it was to let the first process copy its umem-struct into a shared-memory area where the second process could load it from. But because struct xsk_umem is defined in xsk.c it is hidden from the user and I am not able to do something like this:
memcpy(shdm_ptr, umem, sizeof(struct xsk_umem))
I don't know how they expect someone to use the shared umem feature?
So this was discussed on the xdp-newbies mailing list. Reporting here for the record.
It is not recommended to go with a multi-process setup as you are trying to do. Björn says:
Note that if you'd like to do a multi-process setup with shared umem, you: need to have a control process that manages the fill/completion rings, and synchronize between the processes, OR re-mmap the fill/completetion ring from the socket owning the umem in multiple processes and synchronize the access to them. Neither is pleasant.
Honestly, not a setup I'd recommend.
And he adds:
Just for completeness; To setup shared umem:
- create socket 0 and register the umem to this.
- mmap the fr/cr using socket 0
- create socket 1, 2, n and refer to socket 0 for the umem.
So, in a multiprocess solution step 3 would be done in separate processes, and step 2 depending on your application. You'd need to pass socket 0 to the other processes and share the umem memory from the process where socket 0 was created. This is pretty much a threaded solution, given all the shared state.
I advice not taking this path.
(Credits to Björn.)