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

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Message-ID: <20170126165211.GK14167@arm.com>
Date:   Thu, 26 Jan 2017 16:52:12 +0000
From:   Will Deacon <will.deacon@....com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     He Kuang <hekuang@...wei.com>, peterz@...radead.org,
        mingo@...hat.com, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, jolsa@...hat.com,
        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 Thu, Jan 26, 2017 at 10:49:16AM +0900, Masami Hiramatsu wrote:
> 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.

Ok, then do it as two patches then, rather than introduce functionality
along with the renaming.

> > 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 :)

Right, apart from the two howling bugs in the version that was nearly merged
initially :p. I tend to err on the "if it ain't broke, don't fix it" side
of the argument but if you really want the refactoring lets keep it as a
separate change.

Will