x86-64: memcpy performance reduce when running in virtual mechine

[Florian Weimer]

* Shuo Wang:

> The performance of memcpy 1024 has recovered. However, there is performance
> reduce in host. This is test result (cycle):
>
> 	                      memcpy_10	 memcpy_1k	 memcpy_10k	  memcpy_1m	  memcpy_10m
> before backport	             8	         34	        187	        130848	   2325409
> after backport	             8	         34	        182	        515156	   5282603
> Performance improvement	   0.00%	    0.00%	    2.67%	    -293.71%   -127.17%

I think this is expected because the large copies no longer stay within
the cache.  This is required to avoid blowing away the entire cache
contents for such large copies, negatively impacting whole system
performance.  This will of course not show up in a micro-benchmark.

Thanks,
Florian
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