Re: [PATCH v2 net-next 06/16] net: dsa: add a generic procedure for the flow dissector

[Vladimir Oltean <olteanv@gmail.com>]

On Sat, Sep 26, 2020 at 08:05:45PM +0200, Andrew Lunn wrote:
> Given spectra and meltdown etc, jumping through a pointer is expensive
> and we try to avoid it on the hot path. Given most of the taggers are
> going to use the generic version, maybe add a test here, is
> ops->flow_dissect the generic version, and if so, call it directly,
> rather than go through the pointer. Or only set ops->flow_dissect if
> the generic version cannot be used.

Agree about the motivation to eliminate an indirect call if possible.

The situation is as follows:
- Some taggers are before DMAC or before EtherType. These are the vast
  majority, and dsa_tag_generic_flow_dissect works well for them. We can
  keep the .flow_dissect callback as an override, but if this is absent,
  then the flow dissector can call dsa_tag_generic_flow_dissect
  directly.
- Some taggers use tail tags. These don't need any massaging at all. But
  we need to tell the flow dissector to not call
  dsa_tag_generic_flow_dissect if it doesn't find a function pointer.
  I'm thinking about adding another "bool tail_tag" in struct
  dsa_device_ops, for this purpose and not only*.
  *Usually tunnel interfaces need to set dev->needed_headroom for the
  memory allocator. But DSA doesn't request that, and needs to check
  manually in the xmit function if the headroom is large enough to push
  a tag. BUT! tail tags don't need dev->needed_headroom, they need
  dev->needed_tailroom, I think. So I was thinking about adding this
  bool anyway, to distinguish between these 2 cases.

What do you think?