Re: [PATCH 1/1] bpf, docs: Specify twos complement as format for signed integers

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On Tue, Jul 11, 2023 at 12:47 AM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Mon, Jul 10, 2023 at 8:19 PM Will Hawkins <hawkinsw@xxxxxx> wrote:
> >
> > On Mon, Jul 10, 2023 at 11:00 PM Alexei Starovoitov
> > <alexei.starovoitov@xxxxxxxxx> wrote:
> > >
> > > On Mon, Jul 10, 2023 at 2:58 PM Will Hawkins <hawkinsw@xxxxxx> wrote:
> > > >
> > > > In the documentation of the eBPF ISA it is unspecified how integers are
> > > > represented. Specify that twos complement is used.
> > > >
> > > > Signed-off-by: Will Hawkins <hawkinsw@xxxxxx>
> > > > ---
> > > >  Documentation/bpf/instruction-set.rst | 5 +++++
> > > >  1 file changed, 5 insertions(+)
> > > >
> > > > diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst
> > > > index 751e657973f0..63dfcba5eb9a 100644
> > > > --- a/Documentation/bpf/instruction-set.rst
> > > > +++ b/Documentation/bpf/instruction-set.rst
> > > > @@ -173,6 +173,11 @@ BPF_ARSH  0xc0   sign extending dst >>= (src & mask)
> > > >  BPF_END   0xd0   byte swap operations (see `Byte swap instructions`_ below)
> > > >  ========  =====  ==========================================================
> > > >
> > > > +eBPF supports 32- and 64-bit signed and unsigned integers. It does
> > > > +not support floating-point data types. All signed integers are represented in
> > > > +twos-complement format where the sign bit is stored in the most-significant
> > > > +bit.
> > >
> > > Could you point to another ISA document (like x86, arm, ...) that
> > > talks about signed and unsigned integers?
> >
> > Thank you for the reply. I hope that this change is useful. I proposed
> > this change to mimic the documentation of "Numeric Data Types" in
> > Volume 1, Chapter 4 of "Intel® 64 and IA-32 Architectures Software
> > Developer’s Manual" [1].
> >
> > [1] https://www.intel.com/content/www/us/en/developer/articles/technical/intel-sdm.html
>
> I see where you got the inspiration from.
> It's a "software developer's manual". Not an ISA spec.
> But, say, we adopt this form and proceed to create all 500 pages of it.

Though the RISC-V ISA does include information about the integer
representation, your point is well taken.

As I said in a previous message, I am working on setting up the
skeleton for a strawman for a psABI. I will devote my efforts there
where I hope that they can be more useful.

Will

>
> SDM has this to say about pointers:
> "Pointers are addresses of locations in memory.
> In non-64-bit modes, the architecture defines two types of pointers: a
> near pointer and a far pointer. A near pointer is a 32-bit (or 16-bit)
> offset (also called an effective address) within a segment. Near
> pointers are used
> for all memory references in a flat memory model or for references in
> a segmented model where the identity of the segment being accessed is
> implied."
>
> BPF runs on 32-bit and 64-bit CPUs, so if we document signed vs unsigned
> integers we'd have to say a few words about pointers, bitfields and strings
> (just like Intel SDM). Pointers in BPF are clearly lacking docs.
>
> Beyond Vol 1, Chapter 4 there are plenty of other chapters.
> Should we have an equivalent for all of them?
> I think it would be great to have something for all that,
> but dropping a patch or two won't get us there.
> It needs to be a full time commitment with SOW, roadmap, etc.
> I doubt the kernel and/or IETF process can accommodate that.
>
> Saying it differently. What is missing in instruction-set.rst
> from making an IETF standard out of it?
> Does it need a signed vs unsigned SDM-like paragraph?
>
> Let's focus on converting instruction-set.rst into a standard
> as fast as possible and tackle all nice-to-have later.





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