[PATCH net-next v8 3/5] net: dsa: add out-of-band tagging protocol

[Vladimir Oltean]

Hi Jakub,

On Fri, Nov 04, 2022 at 08:05:30PM -0700, Jakub Kicinski wrote:
> On Fri,  4 Nov 2022 18:41:49 +0100 Maxime Chevallier wrote:
> > Add a new tagging protocol based on SKB extensions to convey the
> > information about the destination port to the MAC driver
>
> This is what METADATA_HW_PORT_MUX is for, you shouldn't have
> to allocate a piece of memory for every single packet.

Since this is the model that skb extensions propose and not something
that Maxime invented for this series, I presume that's not such a big
deal? What's more, couldn't this specific limitation of skb extensions
be addressed in a punctual way, via one-time calls to __skb_ext_alloc()
and fast path calls to __skb_ext_set()?

I'm unfamiliar to the concept of destination cache entries and even more
so to the concept of struct dst_entry * carrying metadata. I suppose the
latter were introduced for lack of space in struct sk_buff, to carry
metadata between layers that aren't L3/L4 (where normal dst_entry structs
are used)? What makes metadata dst's preferable to skb extensions?
The latter are more general; AFAIK they can be used between any layer
and any other layer, like for example between RX and TX in the
forwarding path. Side note, I am not exactly clear what are the lifetime
guarantees of a metadata dst entry, and if DSA's use would be 100% safe
(DSA is kind of L3, since it has an ETH_P_XDSA packet_type handler, not
an rx_handler).

More importantly, what happens if a DSA switch is used together with a
SRIOV-capable DSA master which already uses METADATA_HW_PORT_MUX for
PF-VF communication? (if I understood the commit message of 3fcece12bc1b
("net: store port/representator id in metadata_dst") correctly)