Re: [PATCH] net: add initial support for AF_XDP network backend

[Stefan Hajnoczi]

Can multiple VMs share a host netdev by filtering incoming traffic
based on each VM's MAC address and directing it to the appropriate
XSK? If yes, then I think AF_XDP is interesting when SR-IOV or similar
hardware features are not available.

The idea of an AF_XDP passthrough device seems interesting because it
would minimize the overhead and avoid some of the existing software
limitations (mostly in QEMU's networking subsystem) that you
described. I don't know whether the AF_XDP API is suitable or can be
extended to build a hardware emulation interface, but it seems
plausible. When Stefano Garzarella played with io_uring passthrough
into the guest, one of the issues was guest memory translation (since
the guest doesn't use host userspace virtual addresses). I guess
AF_XDP would need an API for adding/removing memory translations or
operate in a mode where addresses are relative offsets from the start
of the umem regions (but this may be impractical if it limits where
the guest can allocate packet payload buffers).

Whether you pursue the passthrough approach or not, making -netdev
af-xdp work in an environment where QEMU runs unprivileged seems like
the most important practical issue to solve.

Stefan