Jesper Dangaard Brouer <brouer@xxxxxxxxxx> writes: > On Wed, 26 Jun 2019 13:52:16 +0200 > Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote: > >> Jesper Dangaard Brouer <brouer@xxxxxxxxxx> writes: >> >> > On Tue, 25 Jun 2019 03:19:22 +0000 >> > "Machulsky, Zorik" <zorik@xxxxxxxxxx> wrote: >> > >> >> On 6/23/19, 7:21 AM, "Jesper Dangaard Brouer" <brouer@xxxxxxxxxx> wrote: >> >> >> >> On Sun, 23 Jun 2019 10:06:49 +0300 <sameehj@xxxxxxxxxx> wrote: >> >> >> >> > This commit implements the basic functionality of drop/pass logic in the >> >> > ena driver. >> >> >> >> Usually we require a driver to implement all the XDP return codes, >> >> before we accept it. But as Daniel and I discussed with Zorik during >> >> NetConf[1], we are going to make an exception and accept the driver >> >> if you also implement XDP_TX. >> >> >> >> As we trust that Zorik/Amazon will follow and implement XDP_REDIRECT >> >> later, given he/you wants AF_XDP support which requires XDP_REDIRECT. >> >> >> >> Jesper, thanks for your comments and very helpful discussion during >> >> NetConf! That's the plan, as we agreed. From our side I would like to >> >> reiterate again the importance of multi-buffer support by xdp frame. >> >> We would really prefer not to see our MTU shrinking because of xdp >> >> support. >> > >> > Okay we really need to make a serious attempt to find a way to support >> > multi-buffer packets with XDP. With the important criteria of not >> > hurting performance of the single-buffer per packet design. >> > >> > I've created a design document[2], that I will update based on our >> > discussions: [2] https://github.com/xdp-project/xdp-project/blob/master/areas/core/xdp-multi-buffer01-design.org >> > >> > The use-case that really convinced me was Eric's packet header-split. >> > >> > >> > Lets refresh: Why XDP don't have multi-buffer support: >> > >> > XDP is designed for maximum performance, which is why certain driver-level >> > use-cases were not supported, like multi-buffer packets (like jumbo-frames). >> > As it e.g. complicated the driver RX-loop and memory model handling. >> > >> > The single buffer per packet design, is also tied into eBPF Direct-Access >> > (DA) to packet data, which can only be allowed if the packet memory is in >> > contiguous memory. This DA feature is essential for XDP performance. >> > >> > >> > One way forward is to define that XDP only get access to the first >> > packet buffer, and it cannot see subsequent buffers. For XDP_TX and >> > XDP_REDIRECT to work then XDP still need to carry pointers (plus >> > len+offset) to the other buffers, which is 16 bytes per extra buffer. >> >> Yeah, I think this would be reasonable. As long as we can have a >> metadata field with the full length + still give XDP programs the >> ability to truncate the packet (i.e., discard the subsequent pages) > > You touch upon some interesting complications already: > > 1. It is valuable for XDP bpf_prog to know "full" length? > (if so, then we need to extend xdp ctx with info) Valuable, quite likely. A hard requirement, probably not (for all use cases). > But if we need to know the full length, when the first-buffer is > processed. Then realize that this affect the drivers RX-loop, because > then we need to "collect" all the buffers before we can know the > length (although some HW provide this in first descriptor). > > We likely have to change drivers RX-loop anyhow, as XDP_TX and > XDP_REDIRECT will also need to "collect" all buffers before the packet > can be forwarded. (Although this could potentially happen later in > driver loop when it meet/find the End-Of-Packet descriptor bit). > > > 2. Can we even allow helper bpf_xdp_adjust_tail() ? > > Wouldn't it be easier to disallow a BPF-prog with this helper, when > driver have configured multi-buffer? Easier, certainly. But then it's even easier to not implement this at all ;) > Or will it be too restrictive, if jumbo-frame is very uncommon and > only enabled because switch infra could not be changed (like Amazon > case). I think it would be preferable to support it; but maybe we can let that depend on how difficult it actually turns out to be to allow it? > Perhaps it is better to let bpf_xdp_adjust_tail() fail runtime? If we do disallow it, I think I'd lean towards failing the call at runtime... -Toke
|