> -----Original Message----- > From: Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> > Sent: Friday, April 26, 2024 12:22 PM > To: Dave Thaler <dthaler1968@xxxxxxxxxxxxxx> > Cc: bpf <bpf@xxxxxxxxxxxxxxx>; bpf@xxxxxxxx; Dave Thaler > <dthaler1968@xxxxxxxxx> > Subject: Re: [PATCH bpf-next] bpf, docs: Clarify PC use in instruction-set.rst > > On Fri, Apr 26, 2024 at 10:11 AM Dave Thaler <dthaler1968@xxxxxxxxxxxxxx> > wrote: > > > > This patch elaborates on the use of PC by expanding the PC acronym, > > explaining the units, and the relative position to which the offset > > applies. > > > > Signed-off-by: Dave Thaler <dthaler1968@xxxxxxxxxxxxxx> > > --- > > Documentation/bpf/standardization/instruction-set.rst | 5 +++++ > > 1 file changed, 5 insertions(+) > > > > diff --git a/Documentation/bpf/standardization/instruction-set.rst > > b/Documentation/bpf/standardization/instruction-set.rst > > index b44bdacd0..5592620cf 100644 > > --- a/Documentation/bpf/standardization/instruction-set.rst > > +++ b/Documentation/bpf/standardization/instruction-set.rst > > @@ -469,6 +469,11 @@ JSLT 0xc any PC += offset if dst < src > signed > > JSLE 0xd any PC += offset if dst <= src signed > > ======== ===== ======= ================================= > > =================================================== > > > > +where 'PC' denotes the program counter, and the offset to increment > > +by is in units of 64-bit instructions relative to the instruction > > +following the jump instruction. Thus 'PC += 1' results in the next > > +instruction to execute being two 64-bit instructions later. > > The last part is confusing. > "two 64-bit instructions later" > I'm struggling to understand that. > Maybe say that 'PC += 1' skips execution of the next insn? If the next instruction is a 64-bit immediate instruction that spans 128 bits, do you need PC += 1 or PC += 2 to skip it? I assumed you'd need PC += 2, in which case "PC += 1" would not skip execution of "the next instruction" but would try to jump into mid instruction, and fail verification. Hence my attempt at "64-bit instruction" wording. Alternate wording suggestions welcome. Dave
