Lorenz Bauer [off-list ref] writes:

On Thu, 23 Sept 2021 at 13:59, Toke Høiland-Jørgensen [off-list ref] wrote:

quoted

I don't think it has to be quite that bleak :)

Specifically, there is no reason to block mb-aware programs from loading
even when the multi-buffer mode is disabled. So a migration plan would
look something like:

...

quoted

2. Start porting all your XDP programs to make them mb-aware, and switch
   their program type as you do. In many cases this is just a matter of
   checking that the programs don't care about packet length. [...]

Porting is only easy if we are guaranteed that the first PAGE_SIZE
bytes (or whatever the current limit is) are available via ->data
without trickery. Otherwise we have to convert all direct packet
access to the new API, whatever that ends up being. It seemed to me
like you were saying there is no such guarantee, and it could be
driver dependent, which is the worst possible outcome imo. This is the
status quo for TC classifiers, which is a great source of hard to
diagnose bugs.

Well, for the changes we're proposing now it will certainly be the case
that the first PAGE_SIZE will always be present. But once we have the
capability, I would expect people would want to do more with it, so we
can't really guarantee this in the future. We could require that any
other use be opt-in at the driver level, I suppose, but not sure if that
would be enough?

-Toke