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

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Date:   Tue, 1 Dec 2020 21:48:36 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Brendan Jackman <jackmanb@...gle.com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Yonghong Song <yhs@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        KP Singh <kpsingh@...omium.org>,
        Florent Revest <revest@...omium.org>,
        LKML <linux-kernel@...r.kernel.org>, Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH v2 bpf-next 02/13] bpf: x86: Factor out emission of REX byte

On Tue, Dec 1, 2020 at 4:12 AM Brendan Jackman <jackmanb@...gle.com> 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@...gle.com>
> > > ---
> > >  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.