serious performance regression in Xen 4.2.4 and 4.2.5

[Andreas Kinzler <ml-ak@posteo.de>]

Hello Xen developers,

since we use Xen for our production systems, I run many tests on Xen 
(stability/performance). One test now uncovered a serious performance 
regression when updating from Xen 4.2.3 to 4.2.x (with x>=4). To 
reproduce run a domU (HVM) and compile a kernel for example ("time make 
-j3"). Below are my results - compilation times more than double!

Is it my mistake or otherwise how could such a bug be unnoticed? Really 
wondering.
If it is really a regression, will it be fixed since the 4.2 branch is 
declared unsupported?

Regards Andreas

Xen 4.2.3, 3.10.44-el7-pvops0-ak3, Xeon E3-1230, compile in HVM
     real    2m58.371s
     user    5m11.580s
     sys     0m12.736s

Xen 4.2.4, 3.10.44-el7-pvops0-ak3, Xeon E3-1230, compile in HVM
     real    6m44.697s    (!!!)
     user    12m4.988s
     sys     0m33.916s

Xen 4.2.5, 3.10.44-el7-pvops0-ak3, Xeon E3-1230, compile in HVM
     real    6m27.998s    (!!!)
     user    11m42.664s
     sys     0m32.700s