On Sat, Sep 26, 2020 at 09:18:15PM +0300, Vladimir Oltean wrote:
> 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?
Sounds good.
Andrew
