On 08/22/2017 10:09 AM, Alexei Starovoitov wrote: > On Tue, Aug 22, 2017 at 08:37:10AM +0200, Jesper Dangaard Brouer wrote: >> >> >>> Once tx-ing netdev added to devmap we can enable xdp on it automatically? >> >> I think you are referring to Gotcha-2 here: > > oops. yes :) > >> >> Second gotcha(2): you cannot TX out a device, unless it also have a >> xdp bpf program attached. (This is an implicit dependency, as the >> driver code need to setup XDP resources before it can ndo_xdp_xmit). >> >> Yes, we should work on improving this situation. Auto enabling XDP >> when a netdev is added to a devmap is a good solution. Currently this >> is tied to loading an XDP bpf_prog. Do you propose loading a dummy >> bpf_prog on the netdev? (then we need to handle 1. not replacing >> existing bpf_prog, 2. on take-down don't remove "later" loaded >> bpf_prog). > > right. these things need to be taken care of. > Technically for ndo_xdp_xmit to work the program doesn't need > to be attached, but the device needs to be in xdp mode with > configured xdp tx rings. > The easiest, of course, is just to document it :) > and may be add some sort of warning that if netdev is added > to devmap and it's not in xdp mode, return warning or error. > When I wrote this I assumed some user space piece could load the "dummy" nop program on devices as needed. It seemed easier than putting semi-complex logic in the kernel to load programs on update_elem, but only if the user hasn't already loaded a program and then unload it but again only if some criteria is met. Then we would have one more kernel path into load/unload BPF programs and would need all the tests and what not. +1 for documenting and userland usability patches. .John
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