linux-kernel - Re: [PATCH v2 04/10] powerpc/traps: Use REG_FMT in show_signal

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Message-Id: <20180730231738.GA20351@kermit-br-ibm-com>
Date:   Mon, 30 Jul 2018 20:17:38 -0300
From:   Murilo Opsfelder Araujo <muriloo@...ux.ibm.com>
To:     LEROY Christophe <christophe.leroy@....fr>
Cc:     linux-kernel@...r.kernel.org,
        "Alastair D'Silva" <alastair@...ilva.org>,
        Andrew Donnellan <andrew.donnellan@....ibm.com>,
        Balbir Singh <bsingharora@...il.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Cyril Bur <cyrilbur@...il.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Joe Perches <joe@...ches.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michael Neuling <mikey@...ling.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Paul Mackerras <paulus@...ba.org>,
        Simon Guo <wei.guo.simon@...il.com>,
        Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>,
        "Tobin C . Harding" <me@...in.cc>, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 04/10] powerpc/traps: Use REG_FMT in show_signal_msg()

Hi, Christophe.

On Mon, Jul 30, 2018 at 06:30:47PM +0200, LEROY Christophe wrote:
> Murilo Opsfelder Araujo <muriloo@...ux.ibm.com> a écrit :
>
> > Hi, Christophe.
> >
> > On Fri, Jul 27, 2018 at 06:40:23PM +0200, LEROY Christophe wrote:
> > > Murilo Opsfelder Araujo <muriloo@...ux.ibm.com> a écrit :
> > >
> > > > Simplify the message format by using REG_FMT as the register format.  This
> > > > avoids having two different formats and avoids checking for MSR_64BIT.
> > >
> > > Are you sure it is what we want ?
> >
> > Yes.
> >
> > > Won't it change the behaviour for a 32 bits app running on a 64bits kernel ?
> >
> > In fact, this changes how many zeroes are prefixed when displaying the
> > registers
> > (%016lx vs. %08lx format).  For example, 32-bits userspace, 64-bits kernel:
>
> Indeed that's what I suspected. What is the real benefit of this change ?
> Why not keep the current format for 32bits userspace ? All those leading
> zeroes are pointless to me.

One of the benefits is simplifying the code by removing some checks.  Another is
deduplicating almost identical format strings in favor of a unified one.

After reading Joe's comment [1], %px seems to be the format we're looking for.
An extract from Documentation/core-api/printk-formats.rst:

  "%px is functionally equivalent to %lx (or %lu). %px is preferred because it
  is more uniquely grep'able."

So I guess we don't need to worry about the format (%016lx vs. %08lx), let's
just use %px, as per the guideline.

[1] https://lore.kernel.org/lkml/26f07092cdde378ebb42c1034badde1b56521c36.camel@perches.com/

Cheers
Murilo