linux-kernel - Re: [PATCH 2/2 v2] perf tools: Enable bpf prologue for arm64

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Message-Id: <20170126104916.971478fa29083cf4ae14fac3@kernel.org>
Date:   Thu, 26 Jan 2017 10:49:16 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Will Deacon <will.deacon@....com>
Cc:     He Kuang <hekuang@...wei.com>, peterz@...radead.org,
        mingo@...hat.com, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, jolsa@...hat.com,
        mhiramat@...nel.org, wangnan0@...wei.com, bintian.wang@...wei.com,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 2/2 v2] perf tools: Enable bpf prologue for arm64

On Wed, 25 Jan 2017 13:32:01 +0000
Will Deacon <will.deacon@....com> wrote:

> On Wed, Jan 25, 2017 at 07:23:11AM +0000, He Kuang wrote:
> > Since HAVE_KPROBES can be enabled in arm64, this patch introduces
> > regs_query_register_offset() to convert register name to offset for
> > arm64, so the BPF prologue feature is ready to use.
> > 
> > This patch also changes the 'dwarfnum' to 'offset' in register table,
> > so the related functions are consistent with x86.
> 
> Wouldn't it be an awful lot simpler just to leave the code as-is, and
> implement regs_query_register_offset in the same way that we implement
> get_arch_regstr but return the dwarfnum?

No, since the offset is not same as dwarfnum.

With this style, the index of array becomes the dwarfnum (the index of
each register defined by DWARF) and the "offset" member means the
byte-offset of the register in (user_)pt_regs. Those should be different.

> I don't really see the point of all the refactoring.

Also, from the maintenance point of view, this rewrite work makes
the code simply similar to x86 implementation, that will be easier to
maintain :)

Thank you,


-- 
Masami Hiramatsu <mhiramat@...nel.org>