Re: [Bpf] [PATCH v3] bpf, docs: Add additional ABI working draft base text

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Wed, Nov 8, 2023 at 3:57 PM Will Hawkins <hawkinsw@obs.cr> wrote:
>
> On Wed, Nov 8, 2023 at 2:51 PM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> >
> > On Wed, Nov 8, 2023 at 2:13 AM Will Hawkins <hawkinsw@obs.cr> wrote:
> > >
> > > On Tue, Nov 7, 2023 at 8:17 PM Alexei Starovoitov
> > > <alexei.starovoitov@gmail.com> wrote:
> > > >
> > > > On Tue, Nov 7, 2023 at 11:56 AM Will Hawkins <hawkinsw@obs.cr> wrote:
> > > > >
> > > > > On Mon, Nov 6, 2023 at 3:38 AM Alexei Starovoitov
> > > > > <alexei.starovoitov@gmail.com> wrote:
> > > > > >
> > > > > > On Sun, Nov 5, 2023 at 4:17 PM Will Hawkins <hawkinsw@obs.cr> wrote:
> > > > > > >
> > > > > > > On Sun, Nov 5, 2023 at 4:51 AM Alexei Starovoitov
> > > > > > > <alexei.starovoitov@gmail.com> wrote:
> > > > > > > >
> > > > > > > > On Fri, Nov 3, 2023 at 2:20 PM Will Hawkins <hawkinsw@obs.cr> wrote:
> > > > > > > > > +
> > > > > > > > > +The ABI is specified in two parts: a generic part and a processor-specific part.
> > > > > > > > > +A pairing of generic ABI with the processor-specific ABI for a certain
> > > > > > > > > +instantiation of a BPF machine represents a complete binary interface for BPF
> > > > > > > > > +programs executing on that machine.
> > > > > > > > > +
> > > > > > > > > +This document is the generic ABI and specifies the parameters and behavior
> > > > > > > > > +common to all instantiations of BPF machines. In addition, it defines the
> > > > > > > > > +details that must be specified by each processor-specific ABI.
> > > > > > > > > +
> > > > > > > > > +These psABIs are the second part of the ABI. Each instantiation of a BPF
> > > > > > > > > +machine must describe the mechanism through which binary interface
> > > > > > > > > +compatibility is maintained with respect to the issues highlighted by this
> > > > > > > > > +document. However, the details that must be defined by a psABI are a minimum --
> > > > > > > > > +a psABI may specify additional requirements for binary interface compatibility
> > > > > > > > > +on a platform.
> > > > > > > >
> > > > > > > > I don't understand what you are trying to say in the above.
> > > > > > > > In my mind there is only one BPF psABI and it doesn't have
> > > > > > > > generic and processor parts. There is only one "processor".
> > > > > > > > BPF is such a processor.
> > > > > > >
> > > > > > > What I was trying to say was that the document here describes a
> > > > > > > generic ABI. In this document there will be areas that are specific to
> > > > > > > different implementations and those would be considered processor
> > > > > > > specific. In other words, the ubpf runtime could define those things
> > > > > > > differently than the rbpf runtime which, in turn, could define those
> > > > > > > things differently than the kernel's implementation.
> > > > > >
> > > > > > I see what you mean. There is only one BPF psABI. There cannot be two.
> > > > > > ubpf can decide not to follow it, but it could only mean that
> > > > > > it's non conformant and not compatible.
> > > > >
> > > > > Okay. That was not how I was structuring the ABI. I thought we had
> > > > > decided that, as the document said, an instantiation of a machine had
> > > > > to
> > > > >
> > > > > 1. meet the gABI
> > > > > 2. specify its requirements vis a vis the psABI
> > > > > 3. (optionally) describe other requirements.
> > > > >
> > > > > If that is not what we decided then we will have to restructure the document.
> > > >
> > > > This abi.rst file is the beginning of "BPF psABI" document.
> > > > We probably should rename it to psabi.rst to avoid confusion.
> > > > See my slides from IETF 118. I hope they explain what "BPF psABI" is for.
> > >
> > > Of course they do! Thank you! My only question: In the language I was
> > > using, I was taking a cue from the System V world where there is a
> > > Generic ABI and a psABI. The Generic ABI applies to all System V
> > > compatible systems and defines certain processor-specific details that
> > > each platform must specify to define a complete ABI. In particular, I
> > > took this language as inspiration
> > >
> > > """
> > > The System V ABI is composed of two basic parts: A generic part of the
> > > specification describes those parts of the interface that remain
> > > constant across all hardware implementations of System V, and a
> > > processor-specific part of the specification describes the parts of
> > > the specification that are specific to a particular processor
> > > architecture. Together, the generic ABI (or gABI) and the processor
> > > specific supplement (or psABI) provide a complete interface
> > > specification for compiled application programs on systems that share
> > > a common hardware architecture.
> > > """
> >
> > I see where you got the inspiration from, but it's not applicable
> > in the BPF case. BPF is such one and only processor.
> > We're not changing nor adding anything to Sys V generic parts.
>
> That was not quite what I was saying. What I started to draft is
> something (yes, modeled after the Sys V (g/ps)ABI) but _brand new_ for
> BPF. I think that is where I have been failing to communicate
> correctly. What I was proposing was inspired by other ABIs but
> completely separate and orthogonal. That is the reason for the
> document speaking of a BPF Machine like:
>
> ABI-conforming BPF Machine Instantiation: A physical or logical realization
>    of a computer system capable of executing BPF programs consistently with the
>    specifications outlined in this document.
>
> because it is a (not necessarily physical) entity that executes BPF
> programs (i.e. a "BPF CPU") for which we are specifying the binary
> compatibility. In other words, the document as it stands is proposing
> a gABI where
>
> the kernel's "BPF CPU" would have its own psABI
> ubpf's "BPF CPU" would have its own psABI

and how would you expect that to work?
psABI is a compiler spec in the first place.
The user would use clang -O2 -target bpf_kernel vs -target bpf_ubpf ?

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