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) 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? 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). Perhaps it is better to let bpf_xdp_adjust_tail() fail runtime? > I think many (most?) use cases will work fine without having access > to the full packet data... I agree. Other people should voice their concerns if they don't agree... -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer
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