Re: [PATCH net-next v8 00/15] Introducing P4TC

Edward Cree
2023-11-23T17:53:46Z

On 23/11/2023 16:30, Jamal Hadi Salim wrote:
> I was hoping not to say anything but my fingers couldnt help themselves:
> So "unoffloadable" means there is a binary blob and this doesnt work
> per your design idea of how it should work?
> Not that it cant be implemented (clearly it has been implemented), it
> is just not how _you_ would implement it? All along I thought this was
> an issue with your hardware.

The kernel doesn't like to trust offload blobs from a userspace compiler,
because it has no way to be sure that what comes out of the compiler
matches the rules/tables/whatever it has in the SW datapath.
It's also a support nightmare because it's basically like each user
compiling their own device firmware. At least normally with device
firmware the driver side is talking to something with narrow/fixed
semantics and went through upstream review, even if the firmware side is
still a black box.
Just to prove I'm not playing favourites: this is *also* a problem with
eBPF offloads like Nanotubes, and I'm not convinced we have a viable
solution yet.

The only way I can see to handle it is something analogous to proof-
carrying code, where the kernel (driver, since the blob is likely to be
wholly vendor-specific) can inspect the binary blob and verify somehow
that (assuming the HW behaves according to its datasheet) it implements
the same thing that exists in SW.
Or simplify the hardware design enough that the compiler can be small
and tight enough to live in-kernel, but that's often impossible.

-ed