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 12:12:49 +0000
From:   Brendan Jackman <jackmanb@...gle.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     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>,
        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 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?

> > + */
> > +static void maybe_emit_rex(u8 **pprog, u32 reg_rm, u32 reg_reg, bool wide)
> 
> could you please keep original names as dst_reg/src_reg instead of reg_rm/reg_reg ?
> reg_reg reads really odd and reg_rm is equally puzzling unless the reader studied
> intel's manual. I didn't. All these new abbreviations are challenging for me.

OK. I originally changed it to use the x86 names because in theory you
could do:

  maybe_emit_rex(&prog, src_reg, dst_reg);

so the names would look backwards when you jump into the function
implementation.

> > +{
> > +	u8 *prog = *pprog;
> > +	int cnt = 0;
> > +
> > +	if (wide)
> 
> what is 'wide' ? Why not to call it 'bool is_alu64' ?

Ack - there's precedent in the file for 'is64' so I'll go with that.