On Tue, 22 Aug 2017 10:17:59 -0700 John Fastabend <john.fastab...@gmail.com> wrote:
> 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. I agree, it is not good to add this kind of logic to the kernel. We would create too many funny race conditions with userspace. > +1 for documenting and userland usability patches. We still don't have a good place for XDP documentation. My own[1] attempt have gotten out-of-sync, and I need to restructure the rst-docs, before I want to propose it for the kernel. [1] https://prototype-kernel.readthedocs.io/en/latest/networking/XDP/index.html I want to create some samples/examples with XDP_REDIRECT, that highlight this property. Maybe the samples/bpf/xdp_redirect{,_map} should automatically attach a XDP bpf_prog to the egress device? -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer
