Probing for native availability of isel from userspace

[malc]

On Sat, 22 Sep 2012, Gabriel Paubert wrote:

> On Sat, Sep 22, 2012 at 02:12:42PM +0400, malc wrote:
> > On Sat, 22 Sep 2012, Segher Boessenkool wrote:
> > 
> > > > Is it possible to determine if _native_ isel is available from userspace
> > > > somehow?
> > > 
> > > Just try to execute one and catch the SIGILL?
> > > 
> > 
> > Unfortunately my kernel emulates ISEL for me in this case, so i don't
> > get any SIGILLs.
> 
> Perform a few isels in a loop between a couple of mftb and measure the 
> shortest time it takes. Any emulation will take tens of timebase ticks,
> hardware implementation will return very small values, perhaps even
> 0 depending on the relationship between core and timebase frequencies.
> 
> I don't remember whether it's necessary inserting an isync between the
> two mftb. I believe that even on the most OOO machines, two mftb in
> a row will not be reordered to the point that the second executes before 
> the first.
> 

Thanks for suggestion, but i'd rather not play with heuristics.

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