On Fri, 28 May 2021 07:35:34 -0700 John Fastabend <john.fastabend@xxxxxxxxx> wrote: > Toke Høiland-Jørgensen wrote: > > John Fastabend <john.fastabend@xxxxxxxxx> writes: > > > > >> > > union and independent set of BTFs are two different things, I'll let > > >> > > you guys figure out which one you need, but I replied how it could > > >> > > look like in CO-RE world > > >> > > > >> > I think a union is sufficient and more aligned with how the > > >> > hardware would actually work. > > >> > > >> Sure. And I think those are two orthogonal concerns. You can start > > >> with a single struct mynic_metadata with union inside it, and later > > >> add the ability to swap mynic_metadata with another > > >> mynic_metadata___v2 that will have a similar union but with a > > >> different layout. > > > > > > Right and then you just have normal upgrade/downgrade problems with > > > any struct. > > > > > > Seems like a workable path to me. But, need to circle back to the > > > what we want to do with it part that Jesper replied to. > > > > So while this seems to be a viable path for getting libbpf to do all the > > relocations (and thanks for hashing that out, I did not have a good grip > > of the details), doing it all in userspace means that there is no way > > for the XDP program to react to changes once it has been loaded. So this > > leaves us with a selection of non-very-attractive options, IMO. I.e., > > we would have to: > > I don't really understand what this means 'having XDP program to > react to changes once it has been loaded.' What would a program look > like thats dynamic? You can always version your metadata and > write programs like this, > > if (meta->version == VERSION1) {do_foo} > else {do_bar} > > And then have a headeer, > > struct meta { > int version; > union ... // union of versions > } > > I fail to see how a program could 'react' dynamically. An agent could > load new programs dynamically into tail call maps of fentry with > the need handlers, which would work as well and avoid unions. > > > > > - have to block any modifications to the hardware config that would > > change the metadata format; this will probably result in irate users > > I'll need a concrete example if I swap out my parser block, I should > also swap out my BPF for my shiny new protocol. I don't see how a > user might write programs for things they've not configured hardware > for yet. Leaving aside knobs like VLAN on/off, VXLAN on/off, and > such which brings the next point. > > > > > - require XDP programs to deal with all possible metadata permutations > > supported by that driver (by exporting them all via a BTF union or > > similar); this means a potential for combinatorial explosion of config > > options and as NICs become programmable themselves I'm not even sure > > if it's possible for the driver to know ahead of time > > I don't see the problem sorry. For current things that exist I can't > think up too many fields vlan, timestamp, checksum(?), pkt_type, > hash maybe. > > For programmable pipelines (P4) then I don't see a problem with > reloading your program or swapping out a program. I don't see the > value of adding a new protocol for example dynamically. Surely > the hardware is going to get hit with a big reset anyways. > > > > > - throw up our hands and just let the user deal with it (i.e., to > > nothing and so require XDP programs to be reloaded if the NIC config > > changes); this is not very friendly and is likely to lead to subtle > > bugs if an XDP program parses the metadata assuming it is in a > > different format than it is > > I'm not opposed to user error causing logic bugs. If I give > users power to reprogram their NICs they should be capabable > of managing a few BPF programs. And if not then its a space > where a distro/vendor should help them with tooling. > > > > > Given that hardware config changes are not just done by ethtool, but > > also by things like running `tcpdump -j`, I really think we have to > > assume that they can be quite dynamic; which IMO means we have to solve > > this as part of the initial design. And I have a hard time seeing how > > this is possible without involving the kernel somehow. > > I guess here your talking about building an skb? Wouldn't it > use whatever logic it uses today to include the timestamp. > This is a bit of an aside from metadata in the BPF program. > > Building timestamps into > skbs doesn't require BPF program to have the data. Or maybe > the point is an XDP variant of tcpdump would like timestamps. > But then it should be in the metadata IMO. It sounds like we are all agreeing that the HW RX timestamp should be stored in the XDP-metadata area right? As I understand, John don't like multiple structs, but want a single struct, so lets create below simple struct that the driver code fills out before calling our XDP-prog: struct meta { u32 timestamp_type; u64 rx_timestamp; u32 rxhash32; u32 version; }; This NIC is configured for PTP, but hardware can only do rx_timestamp for PTP packets (like ixgbe). (Assume both my XDP-prog and PTP userspace prog want to see this HW TS). What should I do as a driver developer to tell XDP-prog that the HW rx_timestamp is not valid for this packet ? 1. Always clear 'rx_timestamp' + 'timestamp_type' for non-PTP packets? 2. or, set version to something else ? I don't like option 1, because it will slowdown the normal none-PTP packets, that doesn't need this timestamp. Now I want to redirect this packet into a veth. The veth device could be running an XDP-prog, that also want to look at this timestamp info. How can the veth XDP-prog know howto interpret metadata area. What if I stored the bpf_id in the version fields in the struct?. (Details: I also need this timestamp info transferred to xdp_frame, because when redirecting into a veth (container) then I want this timestamp set on the SKB to survive. I wonder how can I know what the BTF-layout, guess it would be useful to have btf_id at this point) > > > > Unless I'm missing something? WDYT? > > Distilling above down. I think we disagree on how useful > dynamic programs are because of two reasons. First I don't > see a large list of common attributes that would make the > union approach as painful as you fear. And two, I believe > users who are touching core hardware firmware need to also > be smart enough (or have smart tools) to swap out their > BPF programs in the correct order so as to not create > subtle races. I didn't do it here but if we agree walking > through that program swap flow with firmware update would > be useful. Hmm, I sense we are perhaps talking past each-other(?). I am not talking about firmware upgrades. I'm arguing that enable/disable of HW RX-timestamps will change the XDP-metadata usage dynamically runtime. This is simply a normal userspace program that cause this changes e.g. running 'tcpdump -j'. -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer