From mboxrd@z Thu Jan  1 00:00:00 1970
From: catalin.marinas@arm.com (Catalin Marinas)
Date: Thu, 23 May 2013 09:50:07 +0100
Subject: [PATCH 14/14] ARM: elf: add new hwcap for identifying atomic
 ldrd/strd instructions
In-Reply-To: <20130522180942.GK16361@mudshark.cambridge.arm.com>
References: <1368810473-26070-1-git-send-email-will.deacon@arm.com>
 <1368810473-26070-15-git-send-email-will.deacon@arm.com>
 <20130520141809.GA27473@arm.com>
 <CAL_JsqLSC2QgMdTGtH5G_F5SYjA3Hs--jMmxBES=fqGLwapptQ@mail.gmail.com>
 <20130522084744.GD14322@arm.com>
 <20130522180942.GK16361@mudshark.cambridge.arm.com>
Message-ID: <20130523085006.GA16722@arm.com>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

On Wed, May 22, 2013 at 07:09:42PM +0100, Will Deacon wrote:
> On Wed, May 22, 2013 at 09:47:44AM +0100, Catalin Marinas wrote:
> > On Tue, May 21, 2013 at 07:48:35PM +0100, Rob Herring wrote:
> > > On Mon, May 20, 2013 at 9:18 AM, Catalin Marinas
> > > <catalin.marinas@arm.com> wrote:
> > > > On Fri, May 17, 2013 at 06:07:53PM +0100, Will Deacon wrote:
> > > >> CPUs implementing LPAE have atomic ldrd/strd instructions, meaning that
> > > >> userspace software can avoid having to use the exclusive variants of
> > > >> these instructions if they wish.
> > > >>
> > > >> This patch advertises the atomicity of these instructions via the
> > > >> hwcaps, so userspace can detect this CPU feature.
> > > >>
> > > >> Reported-by: Vladimir Danushevsky <vladimir.danushevsky@oracle.com>
> > > >> Signed-off-by: Will Deacon <will.deacon@arm.com>
> > > > ...
> > > >> +
> > > >> +     /* LPAE implies atomic ldrd/strd instructions */
> > > >> +     vmsa = (read_cpuid_ext(CPUID_EXT_MMFR0) & 0xf) >> 0;
> > > >> +     if (vmsa >= 5)
> > > >> +             elf_hwcap |= HWCAP_LPAE;
> > > >
> > > > As I mentioned in the past, I don't agree with exposing the "LPAE"
> > > > feature to user-space, it's not a feature that user space should care
> > > > about. An atomic double hwcap is better and you can even make this per
> > > > CPU via __v7_proc.
> > > 
> > > How does userspace know whether to install a non-LPAE or LPAE kernel
> > > in a generic way?
> > 
> > This is a valid reason to expose LPAE to user, though elf_hwcap sounds a
> > bit strange.
> 
> In lieu of anything else, do you mind if I continue with the patch as it
> stands?

OK, I give up ;)

-- 
Catalin