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. .John
