Re: [Bpf] Review of draft-thaler-bpf-isa-01

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On Fri, Jul 28, 2023 at 4:32 PM Will Hawkins <hawkinsw@xxxxxx> wrote:
>
> On Thu, Jul 27, 2023 at 9:05 PM Alexei Starovoitov
> <alexei.starovoitov@xxxxxxxxx> wrote:
> >
> > On Wed, Jul 26, 2023 at 12:16 PM Will Hawkins <hawkinsw@xxxxxx> wrote:
> > >
> > > On Tue, Jul 25, 2023 at 2:37 PM Watson Ladd <watsonbladd@xxxxxxxxx> wrote:
> > > >
> > > > On Tue, Jul 25, 2023 at 9:15 AM Alexei Starovoitov
> > > > <alexei.starovoitov@xxxxxxxxx> wrote:
> > > > >
> > > > > On Tue, Jul 25, 2023 at 7:03 AM Dave Thaler <dthaler@xxxxxxxxxxxxx> wrote:
> > > > > >
> > > > > > I am forwarding the email below (after converting HTML to plain text)
> > > > > > to the mailto:bpf@xxxxxxxxxxxxxxx list so replies can go to both lists.
> > > > > >
> > > > > > Please use this one for any replies.
> > > > > >
> > > > > > Thanks,
> > > > > > Dave
> > > > > >
> > > > > > > From: Bpf <bpf-bounces@xxxxxxxx> On Behalf Of Watson Ladd
> > > > > > > Sent: Monday, July 24, 2023 10:05 PM
> > > > > > > To: bpf@xxxxxxxx
> > > > > > > Subject: [Bpf] Review of draft-thaler-bpf-isa-01
> > > > > > >
> > > > > > > Dear BPF wg,
> > > > > > >
> > > > > > > I took a look at the draft and think it has some issues, unsurprisingly at this stage. One is
> > > > > > > the specification seems to use an underspecified C pseudo code for operations vs
> > > > > > > defining them mathematically.
> > > > >
> > > > > Hi Watson,
> > > > >
> > > > > This is not "underspecified C" pseudo code.
> > > > > This is assembly syntax parsed and emitted by GCC, LLVM, gas, Linux Kernel, etc.
> > > >
> > > > I don't see a reference to any description of that in section 4.1.
> > > > It's possible I've overlooked this, and if people think this style of
> > > > definition is good enough that works for me. But I found table 4
> > > > pretty scanty on what exactly happens.
> > >
> > > Hello! Based on Watson's post, I have done some research and would
> > > potentially like to offer a path forward. There are several different
> > > ways that ISAs specify the semantics of their operations:
> > >
> > > 1. Intel has a section in their manual that describes the pseudocode
> > > they use to specify their ISA: Section 3.1.1.9 of The Intel® 64 and
> > > IA-32 Architectures Software Developer’s Manual at
> > > https://cdrdv2.intel.com/v1/dl/getContent/671199
> > > 2. ARM has an equivalent for their variety of pseudocode: Chapter J1
> > > of Arm Architecture Reference Manual for A-profile architecture at
> > > https://developer.arm.com/documentation/ddi0487/latest/
> > > 3. Sail "is a language for describing the instruction-set architecture
> > > (ISA) semantics of processors."
> > > (https://www.cl.cam.ac.uk/~pes20/sail/)
> > >
> > > Given the commercial nature of (1) and (2), perhaps Sail is a way to
> > > proceed. If people are interested, I would be happy to lead an effort
> > > to encode the eBPF ISA semantics in Sail (or find someone who already
> > > has) and incorporate them in the draft.
> >
> > imo Sail is too researchy to have practical use.
> > Looking at arm64 or x86 Sail description I really don't see how
> > it would map to an IETF standard.
> > It's done in a "sail" language that people need to learn first to be
> > able to read it.
> > Say we had bpf.sail somewhere on github. What value does it bring to
> > BPF ISA standard? I don't see an immediate benefit to standardization.
> > There could be other use cases, no doubt, but standardization is our goal.
> >
> > As far as 1 and 2. Intel and Arm use their own pseudocode, so they had
> > to add a paragraph to describe it. We are using C to describe BPF ISA
>
>
> I cannot find a reference in the current version that specifies what
> we are using to describe the operations. I'd like to add that, but
> want to make sure that I clarify two statements that seem to be at
> odds.
>
> Immediately above you say that we are using "C to describe the BPF
> ISA" and further above you say "This is assembly syntax parsed and
> emitted by GCC, LLVM, gas, Linux Kernel, etc."
>
> My own reading is that it is the former, and not the latter. But, I
> want to double check before adding the appropriate statements to the
> Convention section.

It's both. I'm not sure where you see a contradiction.
It's a normal C syntax and it's emitted by the kernel verifier,
parsed by clang/gcc assemblers and emitted by compilers.





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