On Jun 10, Stanislav Fomichev wrote: > On 06/11, Lorenzo Bianconi wrote: > > > Daniel Borkmann <daniel@xxxxxxxxxxxxx> writes: > > > > > [...] > > > >> > > > >> Why not have a new flag for bpf_redirect that transparently stores all > > > >> available metadata? If you care only about the redirect -> skb case. > > > >> Might give us more wiggle room in the future to make it work with > > > >> traits. > > > > > > > > Also q from my side: If I understand the proposal correctly, in order to fully > > > > populate an skb at some point, you have to call all the bpf_xdp_metadata_* kfuncs > > > > to collect the data from the driver descriptors (indirect call), and then yet > > > > again all equivalent bpf_xdp_store_rx_* kfuncs to re-store the data in struct > > > > xdp_rx_meta again. This seems rather costly and once you add more kfuncs with > > > > meta data aren't you better off switching to tc(x) directly so the driver can > > > > do all this natively? :/ > > > > > > I agree that the "one kfunc per metadata item" scales poorly. IIRC, the > > > hope was (back when we added the initial HW metadata support) that we > > > would be able to inline them to avoid the function call overhead. > > > > > > That being said, even with half a dozen function calls, that's still a > > > lot less overhead from going all the way to TC(x). The goal of the use > > > case here is to do as little work as possible on the CPU that initially > > > receives the packet, instead moving the network stack processing (and > > > skb allocation) to a different CPU with cpumap. > > > > > > So even if the *total* amount of work being done is a bit higher because > > > of the kfunc overhead, that can still be beneficial because it's split > > > between two (or more) CPUs. > > > > > > I'm sure Jesper has some concrete benchmarks for this lying around > > > somewhere, hopefully he can share those :) > > > > Another possible approach would be to have some utility functions (not kfuncs) > > used to 'store' the hw metadata in the xdp_frame that are executed in each > > driver codebase before performing XDP_REDIRECT. The downside of this approach > > is we need to parse the hw metadata twice if the eBPF program that is bounded > > to the NIC is consuming these info. What do you think? > > That's the option I was asking about. I'm assuming we should be able > to reuse existing xmo metadata callbacks for this. We should be able > to hide it from the drivers also hopefully. If we move the hw metadata 'store' operations to the driver codebase (running xmo metadata callbacks before performing XDP_REDIRECT), we will parse the hw metadata twice if we attach to the NIC an AF_XDP program consuming the hw metadata, right? One parsing is done by the AF_XDP hw metadata kfunc, and the second one would be performed by the native driver codebase. Moreover, this approach seems less flexible. What do you think? Regards, Lorenzo
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