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From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Brendan Jackman <jackmanb@google.com>
Cc: bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
	Yonghong Song <yhs@fb.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	KP Singh <kpsingh@chromium.org>,
	Florent Revest <revest@chromium.org>,
	LKML <linux-kernel@vger.kernel.org>, Jann Horn <jannh@google.com>
Subject: Re: [PATCH v2 bpf-next 02/13] bpf: x86: Factor out emission of REX byte
Date: Tue, 1 Dec 2020 21:48:36 -0800	[thread overview]
Message-ID: <CAADnVQK63=5XyGxMVe1xuLmQ6NZ5LtCHSVizOUtd1iZ=z747OQ@mail.gmail.com> (raw)
In-Reply-To: <20201201121249.GA2114905@google.com>

On Tue, Dec 1, 2020 at 4:12 AM Brendan Jackman <jackmanb@google.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@google.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.

  reply	other threads:[~2020-12-02  5:49 UTC|newest]

Thread overview: 57+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-27 17:57 [PATCH v2 bpf-next 00/13] Atomics for eBPF Brendan Jackman
2020-11-27 17:57 ` [PATCH v2 bpf-next 01/13] bpf: x86: Factor out emission of ModR/M for *(reg + off) Brendan Jackman
2020-11-29  1:15   ` Alexei Starovoitov
2020-12-01 12:14     ` Brendan Jackman
2020-12-02  5:50       ` Alexei Starovoitov
2020-12-02 10:52         ` Brendan Jackman
2020-12-02 17:35           ` Alexei Starovoitov
2020-11-27 17:57 ` [PATCH v2 bpf-next 02/13] bpf: x86: Factor out emission of REX byte Brendan Jackman
2020-11-29  1:14   ` Alexei Starovoitov
2020-12-01 12:12     ` Brendan Jackman
2020-12-02  5:48       ` Alexei Starovoitov [this message]
2020-12-02 10:54         ` Brendan Jackman
2020-11-27 17:57 ` [PATCH v2 bpf-next 03/13] bpf: x86: Factor out function to emit NEG Brendan Jackman
2020-11-27 17:57 ` [PATCH v2 bpf-next 04/13] bpf: x86: Factor out a lookup table for some ALU opcodes Brendan Jackman
2020-11-27 17:57 ` [PATCH v2 bpf-next 05/13] bpf: Rename BPF_XADD and prepare to encode other atomics in .imm Brendan Jackman
2020-11-28  3:43   ` Yonghong Song
2020-12-01 12:17     ` Brendan Jackman
2020-11-27 17:57 ` [PATCH v2 bpf-next 06/13] bpf: Move BPF_STX reserved field check into BPF_STX verifier code Brendan Jackman
2020-11-27 17:57 ` [PATCH v2 bpf-next 07/13] bpf: Add BPF_FETCH field / create atomic_fetch_add instruction Brendan Jackman
2020-11-28  4:15   ` Yonghong Song
2020-12-01 12:22     ` Brendan Jackman
2020-11-27 17:57 ` [PATCH v2 bpf-next 08/13] bpf: Add instructions for atomic_[cmp]xchg Brendan Jackman
2020-11-28  5:25   ` Yonghong Song
2020-12-01 12:27     ` Brendan Jackman
2020-11-29  1:27   ` Alexei Starovoitov
2020-12-01 12:32     ` Brendan Jackman
2020-11-27 17:57 ` [PATCH v2 bpf-next 09/13] bpf: Pull out a macro for interpreting atomic ALU operations Brendan Jackman
2020-11-27 17:57 ` [PATCH v2 bpf-next 10/13] bpf: Add instructions for atomic[64]_[fetch_]sub Brendan Jackman
2020-11-27 21:39   ` kernel test robot
2020-11-27 21:39   ` [RFC PATCH] bpf: bpf_atomic_alu_string[] can be static kernel test robot
2020-11-28  5:35   ` [PATCH v2 bpf-next 10/13] bpf: Add instructions for atomic[64]_[fetch_]sub Yonghong Song
2020-11-29  1:34     ` Alexei Starovoitov
2020-11-30 17:18       ` Yonghong Song
2020-12-01 12:38         ` Brendan Jackman
2020-12-02  5:55           ` Alexei Starovoitov
2020-12-02 11:19             ` Brendan Jackman
2020-11-27 17:57 ` [PATCH v2 bpf-next 11/13] bpf: Add bitwise atomic instructions Brendan Jackman
2020-11-28  5:39   ` Yonghong Song
2020-11-29  1:36     ` Alexei Starovoitov
2020-11-30 17:20       ` Yonghong Song
2020-11-27 17:57 ` [PATCH v2 bpf-next 12/13] bpf: Add tests for new BPF atomic operations Brendan Jackman
2020-12-01  3:55   ` Yonghong Song
2020-12-01 12:56     ` Brendan Jackman
2020-12-01 17:24       ` Yonghong Song
2020-12-02  2:22   ` Andrii Nakryiko
2020-12-02 12:26     ` Brendan Jackman
2020-11-27 17:57 ` [PATCH v2 bpf-next 13/13] bpf: Document new atomic instructions Brendan Jackman
2020-11-28  5:53 ` [PATCH v2 bpf-next 00/13] Atomics for eBPF Yonghong Song
2020-11-29  1:40   ` Alexei Starovoitov
2020-11-30 17:22     ` Yonghong Song
2020-12-01  3:48       ` Yonghong Song
2020-12-02  2:00         ` Andrii Nakryiko
2020-12-02  5:05           ` Yonghong Song
2020-12-02  5:53             ` John Fastabend
2020-12-02  5:59               ` Andrii Nakryiko
2020-12-02  6:27                 ` John Fastabend
2020-12-02  8:03             ` Yonghong Song

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