[xdp-hints] Basic/Dumb question WAS(Re: Re: XDP-hints via local BTF info

[Jamal Hadi Salim <jhs@mojatatu.com>]

And it goes something like this:

Why does the metadata have to go in the DMA descriptors?
Our experience with the XDP metadata is: you start accessing
that there is a performance penalty (extra cache miss(es)).

Why is the metadata not encapped as part of the data? We
dont have MTU issues on receive since that is entirely a local matter;
meaning the hardware can expand the packet as much as it wants within
the boundaries of alloced DMA buffer space and XDP and any other
subsystem (TC for example) can take advantage of the metadata.
Then extracting metadata becomes a parser issue and you get rid of the
extra cache misses.
Another advantage is: If you make the metadata part the packet data
(and appropriately handle MTU issues) you can pass it around as well
to VMs, containers, other machines,etc

We have done something along these lines using TC IFE, see:
https://legacy.netdevconf.info/0.1/sessions/9.html

The BTF aspect for discoverability is still of value. I am wondering
if we can extend the IFE action to suck in some BTF info so we dont
have to create new kernel code...

cheers,
jamal