Re: Final: Add 32 bit VDSO time function support

["H. Peter Anvin" <hpa@zytor.com>]

On 02/28/2014 06:00 PM, Andy Lutomirski wrote:
> 
> This leads to a potentially interesting question: is rdtsc_barrier()
> actually necessary on UP?  IIRC the point is that, if an
> rdtsc_barrier(); rdtsc in one thread is "before" (in the sense of
> being synchronized by some memory operation) an rdtsc_barrier(); rdtsc
> in another thread, then the first rdtsc needs to return an earlier or
> equal time to the second one.
> 
> I assume that no UP CPU is silly enough to execute two rdtsc
> instructions out of order relative to each other in the absence of
> barriers.  So this is a nonissue on UP.
> 
> On the other hand, suppose that some code does:
> 
> volatile long x = *(something that's not in cache)
> clock_gettime
> 
> I can imagine a modern CPU speculating far enough ahead that the rdtsc
> happens *before* the cache miss.  This won't cause visible
> non-monotonicity as far as I can see, but it might annoy people who
> try to benchmark their code.
> 
> Note: actually making this change might be a bit tricky.  I don't know
> if the alternatives code is smart enough.
> 

Let's put it this way... this is at best a third-order optimization...
let's not worry about it right now.

	-hpa