[PATCH] Report double word access atomicity on LPAE enabled targets through AUXV

[Vladimir Danushevsky]

On Apr 8, 2013, at 10:24 AM, Russell King - ARM Linux wrote:

> On Mon, Apr 08, 2013 at 08:34:28AM -0400, Vladimir Danushevsky wrote:
>> From: Vladimir Danushevsky <vladidan at oracle.com>
> 
> Please don't obfuscate your email address like that - git won't parse it.
> 
>> Hi All,
> 
> We don't include such salutations in the commit message.
> 
I stand corrected for the comments above. Thanks.

>> One of the indirect LPAE features in a single-copy atomicity of LDRD/STRD
>> instructions. This information is useful for some dynamic code generating
>> applications (e.g. JIT compilers) to produce a more efficient access
>> mechanism to atomic/volatile operands.
>> 
>> The feature is part of the design scheme therefore it's not related to the
>> platform configuration i.e. whether CONFIG_ARM_LPAE is enabled or not.
>> 
>> The patch exposes that information through hwcap entry of an Auxiliary
>> Vector.
> 
> I think we're heading towards running out of hwcap bits to represent all
> these different features.  Not there yet, but after this we only have 11
> spare bits.

yes we will eventually run out of the bits, but until that time only user level features need to be exposed through AUXV i.e. LPAE or HYP should not interest an user application. 

> 
> Maybe we need to switch to a more x86-centric method where we report this
> stuff in /proc/cpuinfo, and userspace parses this information out.  If
> glibc already has code in there for x86, maybe we can reuse that?

Doesn't x86 still report first 32 bits of HWCAP in the AUXV?

> 
>> Signed-off-by: Vladimir Danushevsky <vladidan at oracle.com>
> 
> Again, don't obfuscate.

Noted. 

Thanks,
Vlad
> 
>> ---
> 
> diffstat?
> 
>> 
>> --- linux-3.8.4_vanilla/arch/arm/include/uapi/asm/hwcap.h       2013-03-20 16:11:19.000000000 -0400
>> 
>> +++ linux-3.8.4/arch/arm/include/uapi/asm/hwcap.h       2013-03-26 16:32:18.266043699 -0400
> 
> This double-line spacing makes this patch impossible to apply.
> 
>> +        /* check LPAE presence */ 
> 
> If it's worth a comment, it's worth making it a good comment.
>> 
>> +        if (((read_cpuid_ext(CPUID_EXT_MMFR3) >> 28) & 0xf) > 0)
>> 
>> +          elf_hwcap |= HWCAP_ATOMICD;
> 
> We use tabs for indentation, not two spaces.  Also, on the face of it
> ATOMICD isn't obvious what it means.  Maybe HWCAP_LDRDSTRD would be
> better - it's only one character longer and truely reflects what you're
> trying to report - the presence of these two instructions.