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Date:   Mon, 14 Sep 2020 21:36:55 +0300
From:   Ilias Apalodimas <ilias.apalodimas@...aro.org>
To:     Luke Nelson <luke.r.nels@...il.com>
Cc:     Xi Wang <xi.wang@...il.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Will Deacon <will@...nel.org>, bpf <bpf@...r.kernel.org>,
        ardb@...nel.org, naresh.kamboju@...aro.org,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Yauheni Kaliuta <yauheni.kaliuta@...hat.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Zi Shen Lim <zlim.lnx@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Networking <netdev@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Anders Roxell <anders.roxell@...aro.org>,
        Björn Töpel <bjorn.topel@...il.com>
Subject: Re: [PATCH] arm64: bpf: Fix branch offset in JIT

Hi Luke, 

On Mon, Sep 14, 2020 at 11:21:58AM -0700, Luke Nelson wrote:
> On Mon, Sep 14, 2020 at 11:08 AM Xi Wang <xi.wang@...il.com> wrote:
> > I don't think there's some consistent semantics of "offsets" across
> > the JITs of different architectures (maybe it's good to clean that
> > up).  RV64 and RV32 JITs are doing something similar to arm64 with
> > respect to offsets.  CCing Björn and Luke.
> 
> As I understand it, there are two strategies JITs use to keep track of
> the ctx->offset table.
> 
> Some JITs (RV32, RV64, arm32, arm64 currently, x86-32) track the end
> of each instruction (e.g., ctx->offset[i] marks the beginning of
> instruction i + 1).
> This requires care to handle jumps to the first instruction to avoid
> using ctx->offset[-1]. The RV32 and RV64 JITs have special handling
> for this case,
> while the arm32, arm64, and x86-32 JITs appear not to. The arm32 and
> x32 probably need to be fixed for the same reason arm64 does.
> 
> The other strategy is for ctx->offset[i] to track the beginning of
> instruction i. The x86-64 JIT currently works this way.
> This can be easier to use (no need to special case -1) but looks to be
> trickier to construct. This patch changes the arm64 JIT to work this
> way.
> 
> I don't think either strategy is inherently better, both can be
> "correct" as long as the JIT uses ctx->offset in the right way.
> This might be a good opportunity to change the JITs to be consistent
> about this (especially if the arm32, arm64, and x32 JITs all need to
> be fixed anyways).
> Having all JITs agree on the meaning of ctx->offset could help future
> readers debug / understand the code, and could help to someday verify
> the
> ctx->offset construction.
> 
> Any thoughts?

The common strategy does make a lot of sense and yes, both patches will  works 
assuming the ctx->offset ends up being what the JIT engine expects it to be. 
As I mentioned earlier we did consider both, but ended up using the later, 
since as you said, removes the need for handling the special (-1) case.

Cheers
/Ilias

> 
> - Luke

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