Björn Töpel <bjorn.topel@xxxxxxxxx> writes: > On Mon, 15 Apr 2019 at 18:33, Jesper Dangaard Brouer <brouer@xxxxxxxxxx> wrote: >> >> >> On Mon, 15 Apr 2019 13:59:03 +0200 Björn Töpel <bjorn.topel@xxxxxxxxx> wrote: >> >> > Hi, >> > >> > As you probably can derive from the amount of time this is taking, I'm >> > not really satisfied with the design of per-queue XDP program. (That, >> > plus I'm a terribly slow hacker... ;-)) I'll try to expand my thinking >> > in this mail! >> > >> > Beware, it's kind of a long post, and it's all over the place. >> >> Cc'ing all the XDP-maintainers (and netdev). >> >> > There are a number of ways of setting up flows in the kernel, e.g. >> > >> > * Connecting/accepting a TCP socket (in-band) >> > * Using tc-flower (out-of-band) >> > * ethtool (out-of-band) >> > * ... >> > >> > The first acts on sockets, the second on netdevs. Then there's ethtool >> > to configure RSS, and the RSS-on-steriods rxhash/ntuple that can steer >> > to queues. Most users care about sockets and netdevices. Queues is >> > more of an implementation detail of Rx or for QoS on the Tx side. >> >> Let me first acknowledge that the current Linux tools to administrator >> HW filters is lacking (well sucks). We know the hardware is capable, >> as DPDK have an full API for this called rte_flow[1]. If nothing else >> you/we can use the DPDK API to create a program to configure the >> hardware, examples here[2] >> >> [1] https://doc.dpdk.org/guides/prog_guide/rte_flow.html >> [2] https://doc.dpdk.org/guides/howto/rte_flow.html >> >> > XDP is something that we can attach to a netdevice. Again, very >> > natural from a user perspective. As for XDP sockets, the current >> > mechanism is that we attach to an existing netdevice queue. Ideally >> > what we'd like is to *remove* the queue concept. A better approach >> > would be creating the socket and set it up -- but not binding it to a >> > queue. Instead just binding it to a netdevice (or crazier just >> > creating a socket without a netdevice). >> >> Let me just remind everybody that the AF_XDP performance gains comes >> from binding the resource, which allow for lock-free semantics, as >> explained here[3]. >> >> [3] https://github.com/xdp-project/xdp-tutorial/tree/master/advanced03-AF_XDP#where-does-af_xdp-performance-come-from >> > > Yes, but leaving the "binding to queue" to the kernel wouldn't really > change much. It would mostly be that the *user* doesn't need to care > about hardware details. My concern is about "what is a good > abstraction". Can we really guarantee that we will make the right decision from inside the kernel, though? >> >> > The socket is an endpoint, where I'd like data to end up (or get sent >> > from). If the kernel can attach the socket to a hardware queue, >> > there's zerocopy if not, copy-mode. Dito for Tx. >> >> Well XDP programs per RXQ is just a building block to achieve this. >> >> As Van Jacobson explain[4], sockets or applications "register" a >> "transport signature", and gets back a "channel". In our case, the >> netdev-global XDP program is our way to register/program these transport >> signatures and redirect (e.g. into the AF_XDP socket). >> This requires some work in software to parse and match transport >> signatures to sockets. The XDP programs per RXQ is a way to get >> hardware to perform this filtering for us. >> >> [4] http://www.lemis.com/grog/Documentation/vj/lca06vj.pdf >> > > There are a lot of things that are missing to build what you're > describing above. Yes, we need a better way to program the HW from > Linux userland (old topic); What I fail to see is how per-queue XDP is > a way to get hardware to perform filtering. Could you give a > longer/complete example (obviously with non-existing features :-)), so > I get a better view what you're aiming for? > > >> >> > Does a user (control plane) want/need to care about queues? Just >> > create a flow to a socket (out-of-band or inband) or to a netdevice >> > (out-of-band). >> >> A userspace "control-plane" program, could hide the setup and use what >> the system/hardware can provide of optimizations. VJ[4] e.g. suggest >> that the "listen" socket first register the transport signature (with >> the driver) on "accept()". If the HW supports DPDK-rte_flow API we >> can register a 5-tuple (or create TC-HW rules) and load our >> "transport-signature" XDP prog on the queue number we choose. If not, >> when our netdev-global XDP prog need a hash-table with 5-tuple and do >> 5-tuple parsing. >> >> Creating netdevices via HW filter into queues is an interesting idea. >> DPDK have an example here[5], on how to per flow (via ethtool filter >> setup even!) send packets to queues, that endup in SRIOV devices. >> >> [5] https://doc.dpdk.org/guides/howto/flow_bifurcation.html >> >> >> > Do we envison any other uses for per-queue XDP other than AF_XDP? If >> > not, it would make *more* sense to attach the XDP program to the >> > socket (e.g. if the endpoint would like to use kernel data structures >> > via XDP). >> >> As demonstrated in [5] you can use (ethtool) hardware filters to >> redirect packets into VFs (Virtual Functions). >> >> I also want us to extend XDP to allow for redirect from a PF (Physical >> Function) into a VF (Virtual Function). First the netdev-global >> XDP-prog need to support this (maybe extend xdp_rxq_info with PF + VF >> info). Next configure HW filter to queue# and load XDP prog on that >> queue# that only "redirect" to a single VF. Now if driver+HW supports >> it, it can "eliminate" the per-queue XDP-prog and do everything in HW. >> > > Again, let's try to be more concrete! So, one (non-existing) mechanism > to program filtering to HW queues, and then attaching a per-queue > program to that HW queue, which can in some cases be elided? I'm not > opposing the idea of per-queue, I'm just trying to figure out > *exactly* what we're aiming for. > > My concern is, again, mainly that is a queue abstraction something > we'd like to introduce to userland. It's not there (well, no really > :-)) today. And from an AF_XDP userland perspective that's painful. > "Oh, you need to fix your RSS hashing/flow." E.g. if I read what > Jonathan is looking for, it's more of something like what Jiri Pirko > suggested in [1] (slide 9, 10). > > Hey, maybe I just need to see the fuller picture. :-) AF_XDP is too > tricky to use from XDP IMO. Per-queue XDP program would *optimize* > AF_XDP, but not solving the filtering. Maybe starting in the > filtering/metadata offload path end of things, and then see what we're > missing. > >> >> > If we'd like to slice a netdevice into multiple queues. Isn't macvlan >> > or similar *virtual* netdevices a better path, instead of introducing >> > yet another abstraction? >> >> XDP redirect a more generic abstraction that allow us to implement >> macvlan. Except macvlan driver is missing ndo_xdp_xmit. Again first I >> write this as global-netdev XDP-prog, that does a lookup in a BPF-map. >> Next I configure HW filters that match the MAC-addr into a queue# and >> attach simpler XDP-prog to queue#, that redirect into macvlan device. >> > > Just for context; I was thinking something like macvlan with > ndo_dfwd_add/del_station functionality. "A virtual interface that is > simply is a view of a physical". A per-queue program would then mean > "create a netdev for that queue". My immediate reaction is that I kinda like this model from an API PoV; not sure what it would take to get there, though? When you say 'something like macvlan', you do mean we'd have to add something completely new, right? -Toke
