From mboxrd@z Thu Jan  1 00:00:00 1970
From: catalin.marinas@arm.com (Catalin Marinas)
Date: Mon, 20 May 2013 15:18:09 +0100
Subject: [PATCH 14/14] ARM: elf: add new hwcap for identifying atomic
 ldrd/strd instructions
In-Reply-To: <1368810473-26070-15-git-send-email-will.deacon@arm.com>
References: <1368810473-26070-1-git-send-email-will.deacon@arm.com>
 <1368810473-26070-15-git-send-email-will.deacon@arm.com>
Message-ID: <20130520141809.GA27473@arm.com>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

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.

-- 
Catalin