[PATCH] eal/x86: cache queried CPU flags

[Wathsala Wathawana Vithanage]

> > Hi Bruce,
> > Why x86/rte_cpuflags.c is not using rte_getauxval() ?
> 
> I don't see any such DPDK function, unless I'm missing something? I do see a
> linux function which includes hw capability flags. I suspect the reasons we
> don't use that are:
> 
> 1. Lack of awareness of it (at least on my end! :-)) 2. Originally when we added
> flags for new instruction sets we may have
>    needed to support their detection on platforms where the kernel/libc was
>    too old to be aware of them.
> 3. It's linux only, so we'd still need these cpuid calls for other OS's
>    anyway.
> 
> Therefore, I don't see any benefit in using that function over what we do now.
> Is there some benefit that you see that makes you think it would be
> worthwhile switching?
> 

Agree, all these are good reasons to avoid it. 
getauxval() reads these values from process stack (ELF loader places it there), so it's
a relatively fast way of finding cpu features. I'm wondering if a similar mechanism is
available in FreeBSD and Windows. 
If so, it might be beneficial to have a common API for such functions.
But as you mentioned, if the kernel isn't aware of the feature it may fail.