From mboxrd@z Thu Jan 1 00:00:00 1970 From: catalin.marinas@arm.com (Catalin Marinas) Date: Wed, 22 May 2013 09:47:44 +0100 Subject: [PATCH 14/14] ARM: elf: add new hwcap for identifying atomic ldrd/strd instructions In-Reply-To: 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> Message-ID: <20130522084744.GD14322@arm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Tue, May 21, 2013 at 07:48:35PM +0100, Rob Herring wrote: > On Mon, May 20, 2013 at 9:18 AM, Catalin Marinas > 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 > >> Signed-off-by: Will Deacon > > ... > >> + > >> + /* 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. -- Catalin