Messages in this thread | | | Subject | Re: [PATCH net-next 0/2] virtio-net: re enable XDP_REDIRECT for mergeable buffer | From | Jason Wang <> | Date | Fri, 2 Mar 2018 12:17:44 +0800 |
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On 2018年03月01日 22:16, Jesper Dangaard Brouer wrote: > On Thu, 1 Mar 2018 21:15:36 +0800 > Jason Wang <jasowang@redhat.com> wrote: > >> On 2018年03月01日 18:35, Jesper Dangaard Brouer wrote: >>> On Thu, 1 Mar 2018 17:23:37 +0800 >>> Jason Wang <jasowang@redhat.com> wrote: >>> >>>> On 2018年03月01日 17:10, Jesper Dangaard Brouer wrote: >>>>> On Thu, 1 Mar 2018 11:19:03 +0800 >>>>> Jason Wang <jasowang@redhat.com> wrote: >>>>> >>>>>> This series tries to re-enable XDP_REDIRECT for mergeable buffer which >>>>>> was removed since commit 7324f5399b06 ("virtio_net: disable >>>>>> XDP_REDIRECT in receive_mergeable() case"). Main concerns are: >>>>>> >>>>>> - not enough tailroom was reserved which breaks cpumap >>>>> To address this at a more fundamental level, I would suggest that we/you >>>>> instead extend XDP to know it's buffers "frame" size/end. (The >>>>> assumption use to be, xdp_buff->data_hard_start + PAGE_SIZE, but >>>>> ixgbe+virtio_net broke that assumption). >>>>> >>>>> It should actually be fairly easy to implement: >>>>> * Simply extend xdp_buff with a "data_hard_end" pointer. >>>> Right, and then cpumap can warn and drop packets with insufficient >>>> tailroom. >>>> >>>> But it should be a patch on top of this I think. >>> Hmmm, not really. If we/you instead fix the issue of XDP doesn't know >>> the end/size of the frame, then we don't need this mixed XDP >>> generic/native code path mixing. >> I know this but I'm still a little bit confused. According to the commit >> log of 7324f5399b06 ("virtio_net: disable XDP_REDIRECT in >> receive_mergeable() case"), you said: >> >> """ >> The longer explaination is that receive_mergeable() tries to >> work-around and satisfy these XDP requiresments e.g. by having a >> function xdp_linearize_page() that allocates and memcpy RX buffers >> around (in case packet is scattered across multiple rx buffers). This >> does currently satisfy XDP_PASS, XDP_DROP and XDP_TX (but only because >> we have not implemented bpf_xdp_adjust_tail yet). >> """ >> >> So I consider the tailroom is a must for the (future) tail adjustment. > That is true, BUT implementing the "data_hard_end" extension is a > pre-requisite. It will also be to catch the issue of too little > tail-room if/when implementing bpf_xdp_adjust_tail(). > > It is of-cause a "nice-to-have", to fix this virtio_net driver's > receive_mergeable() call to have enough tail-room, but I don't see it > as a solution to the fundamental problem. > > >>> You could re-enable native redirect, and push the responsibility to >>> cpumap for detecting this too-small frame "missing tailroom" (and avoid >>> crashing...). (If we really want to support this, cpumap could fallback >>> to dev_alloc_skb, and handle it gracefully). >>> >> Right but it will be slower than build_skb(). > True, but bad argument in this context, as you are already using a > similar function call napi_alloc_skb(). And it will be even slower to > call generic-XDP code path. >
Well, there's no generic skb implementation for cpumap redirection so I think we're talking about native XDP for cpumap, In this case, we won't even use napi_alloc_skb().
Thanks
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