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. 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.
