x86 CPU features detection for applications (and AMX)

[Peter Zijlstra]

On Mon, Jun 28, 2021 at 10:23:47AM -0700, Thiago Macieira wrote:
> On Monday, 28 June 2021 10:11:16 PDT Peter Zijlstra wrote:
> > > Consequence: CPU feature checking is done *very* early, often before
> > > main().
> > For the linker based ones, yes. IIRC the ifunc() attribute is
> > particularly useful here.
> 
> Exactly. ifunc was designed for this exact purpose. And hence the fact that 
> CPUID initialisation will be done very, very early.
> 
> Anyway, if the AMX state is a sticky "set once per process", it's likely going 
> to get set early for every process that *may* use AMX. And this is assuming we 
> do the library right and only set it if has AMX code at all, instead of all 
> the time.

This, AFAIU. If the ifunc() resolver finds we haz AMX it can do the
prctl() and on success pick the AMX routine.

Assuming of course, that if a program links with a library that supports
AMX, it will actually end up using it.