Re: [PATCH net-next v2 3/6] net: bridge: add 802.1Qat stream reservation admission control

[Luke Howard]

> So it sounds like you first need to work on the software
> implementation and expand the simplified version with the features you
> need. You can then add support to accelerate this by offloading it to
> the hardware.

Agreed.

The definition of Dynamic Reservation Entries in 802.1Q (clause 8.8.7) might support the addition of a new MDB (or even FDB) entry state to the kernel:

- add MDB_DYNAMIC_RESERVATION (a state, not a flag);
- the software bridge only _classifies_ packets against  MDB_DYNAMIC_RESERVATION entries, and only when MDB is authoritative. Classification sets dynamic_reservation_hit on tc_skb_ext;
- dynamic_reservation_hit is visible to the flow dissector so can be used for policy enforcement.

Advantages:

- the new MDB state maps well to 802.1Q;
- minimal changes to bridge;
- actual policy (reclassify, drop, etc) is left to the user;
- doesn’t require a new tc-flower entry for each stream DA;
- entry state maps 1:1 to mv88e6xxx AVB_NRL ATU EntryState.

Disadvantages:

- no unicast support, although potentially can be extended (there are some subtleties);
- mv88e6xxx support would either require rich enough tc-flower support in TCAM, intercepting TCA_FLOWER_DYNAMIC_RESERVATION_HIT and mapping to native AVB admission control, or a per-port devlink parameter (not so nice).

This is implemented and working with the software bridge, I still haven’t quite figured out the right mapping for mv88e6xxx.

Luke

PS. I previously incorrectly asserted that 802.1Q required dropping frames with AVB/SRP PCPs but without valid dynamic reservation entries. 802.1Q discussions priority mapping in clause 6.9.4 for traffic from SRP boundary ports (those not participating in SRP). In practice I think this should all be policy, e.g. you might want to reprioritise valid DSCP traffic from within a SRP domain, or drop instead of reprioritise, etc.