On Wed, Feb 16, 2022 at 17:28, Nikolay Aleksandrov <nikolay@xxxxxxxxxx> wrote: > On 16/02/2022 15:29, Tobias Waldekranz wrote: >> The bridge has had per-VLAN STP support for a while now, since: >> >> https://lore.kernel.org/netdev/20200124114022.10883-1-nikolay@xxxxxxxxxxxxxxxxxxx/ >> >> The current implementation has some problems: >> >> - The mapping from VLAN to STP state is fixed as 1:1, i.e. each VLAN >> is managed independently. This is awkward from an MSTP (802.1Q-2018, >> Clause 13.5) point of view, where the model is that multiple VLANs >> are grouped into MST instances. >> >> Because of the way that the standard is written, presumably, this is >> also reflected in hardware implementations. It is not uncommon for a >> switch to support the full 4k range of VIDs, but that the pool of >> MST instances is much smaller. Some examples: >> >> Marvell LinkStreet (mv88e6xxx): 4k VLANs, but only 64 MSTIs >> Marvell Prestera: 4k VLANs, but only 128 MSTIs >> Microchip SparX-5i: 4k VLANs, but only 128 MSTIs >> >> - By default, the feature is enabled, and there is no way to disable >> it. This makes it hard to add offloading in a backwards compatible >> way, since any underlying switchdevs have no way to refuse the >> function if the hardware does not support it >> >> - The port-global STP state has precedence over per-VLAN states. In >> MSTP, as far as I understand it, all VLANs will use the common >> spanning tree (CST) by default - through traffic engineering you can >> then optimize your network to group subsets of VLANs to use >> different trees (MSTI). To my understanding, the way this is >> typically managed in silicon is roughly: >> >> Incoming packet: >> .----.----.--------------.----.------------- >> | DA | SA | 802.1Q VID=X | ET | Payload ... >> '----'----'--------------'----'------------- >> | >> '->|\ .----------------------------. >> | +--> | VID | Members | ... | MSTI | >> PVID -->|/ |-----|---------|-----|------| >> | 1 | 0001001 | ... | 0 | >> | 2 | 0001010 | ... | 10 | >> | 3 | 0001100 | ... | 10 | >> '----------------------------' >> | >> .-----------------------------' >> | .------------------------. >> '->| MSTI | Fwding | Lrning | >> |------|--------|--------| >> | 0 | 111110 | 111110 | >> | 10 | 110111 | 110111 | >> '------------------------' >> >> What this is trying to show is that the STP state (whether MSTP is >> used, or ye olde STP) is always accessed via the VLAN table. If STP >> is running, all MSTI pointers in that table will reference the same >> index in the STP stable - if MSTP is running, some VLANs may point >> to other trees (like in this example). >> >> The fact that in the Linux bridge, the global state (think: index 0 >> in most hardware implementations) is supposed to override the >> per-VLAN state, is very awkward to offload. In effect, this means >> that when the global state changes to blocking, drivers will have to >> iterate over all MSTIs in use, and alter them all to match. This >> also means that you have to cache whether the hardware state is >> currently tracking the global state or the per-VLAN state. In the >> first case, you also have to cache the per-VLAN state so that you >> can restore it if the global state transitions back to forwarding. >> >> This series adds support for an arbitrary M:N mapping of VIDs to >> MSTIs, proposing one solution to the first issue. An example of an >> offload implementation for mv88e6xxx is also provided. Offloading is >> done on a best-effort basis, i.e. notifications of the relevant events >> are generated, but there is no way for the user to see whether the >> per-VLAN state has been offloaded or not. There is also no handling of >> the relationship between the port-global state the the per-VLAN ditto. >> >> If I was king of net/bridge/*, I would make the following additional >> changes: >> >> - By default, when a VLAN is created, assign it to MSTID 0, which >> would mean that no per-VLAN state is used and that packets belonging >> to this VLAN should be filtered according to the port-global state. >> >> This way, when a VLAN is configured to use a separate tree (setting >> a non-zero MSTID), an underlying switchdev could oppose it if it is >> not supported. >> >> Obviously, this adds an extra step for existing users of per-VLAN >> STP states and would thus not be backwards compatible. Maybe this >> means that that is impossible to do, maybe not. >> >> - Swap the precedence of the port-global and the per-VLAN state, >> i.e. the port-global state only applies to packets belonging to >> VLANs that does not make use of a per-VLAN state (MSTID != 0). >> >> This would make the offloading much more natural, as you avoid all >> of the caching stuff described above. >> >> Again, this changes the behavior of the kernel so it is not >> backwards compatible. I suspect that this is less of an issue >> though, since my guess is that very few people rely on the old >> behavior. >> >> Thoughts? >> > > Interesting! Would adding a new (e.g. vlan_mst_enable) option which changes the behaviour > as described help? It can require that there are no vlans present to change > similar to the per-port vlan stats option. Great idea, I did not know that that's how vlan stats worked. I will definitely look into it, thanks! > Also based on that option you can alter > how the state checks are performed. For example, you can skip the initial port state > check, then in br_vlan_allowed_ingress() you can use the port state if vlan filtering > is disabled and mst enabled and you can avoid checking it altogether if filter && mst > are enabled then always use the vlan mst state. Similar changes would have to happen > for the egress path. Since we are talking about multiple tests the new MST logic can > be hidden behind a static key for both br_handle_frame() and later stages. Makes sense. So should we keep the current per-VLAN state as-is then? And bolt the MST on to the side? I.e. should `struct net_bridge_vlan` both have `u8 state` for the current implementation _and_ a `struct br_vlan_mst *` that is populated for VLANs tied to a non-zero MSTI? > This set needs to read a new cache line to fetch mst ptr for all packets in the vlan fast-path, > that is definitely undesirable. Please either cache that state in the vlan and update it when > something changes, or think of some way which avoids that cache line in fast-path. > Alternative would be to make that cache line dependent on the new option, so it's needed > only when mst feature is enabled. If we go with the approach I suggested above, then the current `u8 state` on `struct net_bridge_vlan` could be that cache, right? With the current implementation, it is set directly - in the new MST mode all grouped VLANs would have their states updated when updating the MSTI's state.
