On Mon, Apr 04, 2016 at 09:07:03AM -0700, John Fastabend wrote: > On 16-04-04 08:29 AM, Brenden Blanco wrote: > > On Mon, Apr 04, 2016 at 05:12:27PM +0200, Jesper Dangaard Brouer wrote: > >> On Mon, 4 Apr 2016 11:09:57 -0300 > >> Tom Herbert <t...@herbertland.com> wrote: > >> > >>> On Mon, Apr 4, 2016 at 10:36 AM, Daniel Borkmann <dan...@iogearbox.net> > >>> wrote: > >>>> On 04/04/2016 03:07 PM, Jesper Dangaard Brouer wrote: > >>>>> > >>>>> On Mon, 04 Apr 2016 10:49:09 +0200 Daniel Borkmann > >>>>> <dan...@iogearbox.net> > >>>>> wrote: > >>>>>> > >>>>>> On 04/02/2016 03:21 AM, Brenden Blanco wrote: > >>>>>>> > >>>>>>> Add a new bpf prog type that is intended to run in early stages of the > >>>>>>> packet rx path. Only minimal packet metadata will be available, hence > >>>>>>> a > >>>>>>> new > >>>>>>> context type, struct xdp_metadata, is exposed to userspace. So far > >>>>>>> only > >>>>>>> expose the readable packet length, and only in read mode. > >>>>>>> > >>>>>>> The PHYS_DEV name is chosen to represent that the program is meant > >>>>>>> only > >>>>>>> for physical adapters, rather than all netdevs. > >>>>>>> > >>>>>>> While the user visible struct is new, the underlying context must be > >>>>>>> implemented as a minimal skb in order for the packet load_* > >>>>>>> instructions > >>>>>>> to work. The skb filled in by the driver must have skb->len, > >>>>>>> skb->head, > >>>>>>> and skb->data set, and skb->data_len == 0. > >>>>>>> > >>>>> [...] > >>>>>> > >>>>>> > >>>>>> Do you plan to support bpf_skb_load_bytes() as well? I like using > >>>>>> this API especially when dealing with larger chunks (>4 bytes) to > >>>>>> load into stack memory, plus content is kept in network byte order. > >>>>>> > >>>>>> What about other helpers such as bpf_skb_store_bytes() et al that > >>>>>> work on skbs. Do you intent to reuse them as is and thus populate > >>>>>> the per cpu skb with needed fields (faking linear data), or do you > >>>>>> see larger obstacles that prevent for this? > >>>>> > >>>>> > >>>>> Argh... maybe the minimal pseudo/fake SKB is the wrong "signal" to send > >>>>> to users of this API. > >>>>> > >>>>> The hole idea is that an SKB is NOT allocated yet, and not needed at > >>>>> this level. If we start supporting calling underlying SKB functions, > >>>>> then we will end-up in the same place (performance wise). > >>>> > >>>> > >>>> I'm talking about the current skb-related BPF helper functions we have, > >>>> so the question is how much from that code we have we can reuse under > >>>> these constraints (obviously things like the tunnel helpers are a > >>>> different > >>>> story) and if that trade-off is acceptable for us. I'm also thinking > >>>> that, for example, if you need to parse the packet data anyway for a drop > >>>> verdict, you might as well pass some meta data (that is set in the real > >>>> skb later on) for those packets that go up the stack. > >>> > >>> Right, the meta data in this case is an abstracted receive descriptor. > >>> This would include items that we get in a device receive descriptor > >>> (computed checksum, hash, VLAN tag). This is purposely a small > >>> restricted data structure. I'm hoping we can minimize the size of this > >>> to not much more than 32 bytes (including pointers to data and > >>> linkage). > >> > >> I agree. > >> > >>> How this translates to skb to maintain compatibility is with BPF > >>> interesting question. One other consideration is that skb's are kernel > >>> specific, we should be able to use the same BPF filter program in > >>> userspace over DPDK for instance-- so an skb interface as the packet > >>> abstraction might not be the right model... > >> > >> I agree. I don't think reusing the SKB data structure is the right > >> model. We should drop the SKB pointer from the API. > >> > >> As Tom also points out, making the BPF interface independent of the SKB > >> meta-data structure, would also make the eBPF program more generally > >> applicable. > > The initial approach that I tried went down this path. Alexei advised > > that I use the pseudo skb, and in the future the API between drivers and > > bpf can change to adopt non-skb context. The only user facing ABIs in > > this patchset are the IFLA, the xdp_metadata struct, and the name of the > > new enum. > > > > The reason to use a pseudo skb for now is that there will be a fair > > amount of churn to get bpf jit and interpreter to understand non-skb > > context in the bpf_load_pointer() code. I don't see the need for > > requiring that for this patchset, as it will be internal-only change > > if/when we use something else. > > Another option would be to have per driver JIT code to patch up the > skb read/loads with descriptor reads and metadata. From a strictly > performance stand point it should be better than pseudo skbs.
I considered (and implemented) this as well, but there my problem was that I needed to inform the bpf() syscall at BPF_PROG_LOAD time which ifindex to look at for fixups, so I had to add a new ifindex field to bpf_attr. Then during verification I had to use a new ndo to get the driver-specific offsets for its particular descriptor format. It seemed kludgy. > > .John
