Re: [PATCH v2 bpf-next 02/13] bpf: x86: Factor out emission of REX byte

[Brendan Jackman]

On Tue, Dec 01, 2020 at 09:48:36PM -0800, Alexei Starovoitov wrote:
> On Tue, Dec 1, 2020 at 4:12 AM Brendan Jackman <jackmanb@xxxxxxxxxx> wrote:
> >
> > On Sat, Nov 28, 2020 at 05:14:05PM -0800, Alexei Starovoitov wrote:
> > > On Fri, Nov 27, 2020 at 05:57:27PM +0000, Brendan Jackman wrote:
> > > > The JIT case for encoding atomic ops is about to get more
> > > > complicated. In order to make the review & resulting code easier,
> > > > let's factor out some shared helpers.
> > > >
> > > > Signed-off-by: Brendan Jackman <jackmanb@xxxxxxxxxx>
> > > > ---
> > > >  arch/x86/net/bpf_jit_comp.c | 39 ++++++++++++++++++++++---------------
> > > >  1 file changed, 23 insertions(+), 16 deletions(-)
> > > >
> > > > diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> > > > index 94b17bd30e00..a839c1a54276 100644
> > > > --- a/arch/x86/net/bpf_jit_comp.c
> > > > +++ b/arch/x86/net/bpf_jit_comp.c
> > > > @@ -702,6 +702,21 @@ static void emit_modrm_dstoff(u8 **pprog, u32 r1, u32 r2, int off)
> > > >     *pprog = prog;
> > > >  }
> > > >
> > > > +/*
> > > > + * Emit a REX byte if it will be necessary to address these registers
> > >
> > > What is "REX byte" ?
> > > May be rename it to maybe_emit_mod() ?
> >
> > Er, this is the REX prefix as described in
> > https://wiki.osdev.org/X86-64_Instruction_Encoding#REX_prefix
> >
> > Would maybe_emit_mod be accurate? In my mind "mod" is a field in the
> > ModR/M byte which comes _after_ the opcode. Before developing this
> > patchset I knew almost nothing about x86, so maybe I'm missing something
> > about the general terminology?
> 
> I wrote the JIT without looking into the manual and without studying
> the terminology.
> Why? Because it was not necessary. I still don't see a reason why
> that obscure terminology needs to be brought in into the code.
> 'mod' to me is a 'modifier'. Nothing to do with intel's modrm thing.

OK, calling it maybe_emit_mod(pprog, dst_reg, src_reg)