Re: [Bpf] BPF ISA conformance groups

[Christoph Hellwig <hch@infradead.org>]

On Thu, Dec 14, 2023 at 11:44:37AM -0600, David Vernet wrote:
> > > Why else would they be asking for a standard if not to
> > > have some guidelines of what to implement?
> > 
> > Excellent question. I don't know why nvme folks need a standard.
> > Lack of standard didn't stop netronome.
> 
> Christoph? Any chance you can shed some light here?

netronome is a single vendor implementation.  You write for their
device and the standard is what they accept.  NVMe is an open,
multi-vendor standard.  You need to be able to write your code against
the spec and run it on all devices (that implement the required
features).  NVMe also needs another open standard as the reference as
it just can't point to a void.

> I agree that there's value in instructions having specific meaning and
> encodings, but my worry is that (for device offload) the value would be
> minimized quite a bit if a developer writing a BPF offload program
> doesn't also have some knowledge or guarantee of what instructions
> vendors have actually implemented.

Absolutely.

> If we were to do away with conformance groups, then I as a BPF user
> would have the guarantee: "Any hw device which happens to implement the
> instructions in my program will behave in a predictable way". If that
> user doesn't know what instructions it can count on being actually
> available in devices, then they're going to end up just implementing the
> program for a single device anyways. At that point, how useful was it
> really to standardize on the semantics of the instructions? That user
> just as soon could have read the specifications for the device and
> implemented the prog according to the semantics that the vendor decided
> were most appropriate for them.

We need the concept in the spec just to allow future extensability.