[PATCH 14/14] ARM: elf: add new hwcap for identifying atomic ldrd/strd instructions

[Rob Herring]

On Mon, May 20, 2013 at 9:18 AM, Catalin Marinas
<catalin.marinas at 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 at oracle.com>
>> Signed-off-by: Will Deacon <will.deacon at 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?

Rob