Right now all HyperV enlightenment for Windows guests in RHEV are disabled. As of this bug#1163828. RHEV 3.6 is based RHEL 7 hypervisors and bug#1063124 affects only RHEL 6, we should enable this flag by default and let Windows guests to enjoy the improvements brought by HyperV enlightments to kvm. For RHEL6 - we can either disable this on vdsm side (to never enable hv_relaxed flag), or make this option configurable on the engine side.
Here is some more background on this: We have absolutely different levels of supported "hv_" features in rhel6.x and rhel7.x. hv_relaxed is the only supported Hyper-V feature in rhel6.x. This flag is a must for Win7/WS2008 on any RHEL version. Win8 and more recent Windows versions can operate normally without this flag turned on. And this is important, taking into account that on some CPU configurations activating hv_relaxed flag can lead to the bug described in #1063124. rhel7.x provides a better Hyper-V support, and seems to be free of the problem mentioned in #1063124. In any case, disabling hv_relaxed for Win8 and higher on rhel7.x should not affect the set other Hyper-V optimizations (like spinlocks, vapic and reference time) supported by KVM and explicitly enabled in QEMU command line.
Verified: rhevm-3.6.6.2-0.1.el6 sanlock-3.2.4-2.el7_2.x86_64 libvirt-client-1.2.17-13.el7_2.4.x86_64 qemu-kvm-rhev-2.3.0-31.el7_2.11.x86_64 vdsm-4.17.28-0.el7ev.noarch
Since the problem described in this bug report should be resolved in a recent advisory, it has been closed with a resolution of ERRATA. For information on the advisory, and where to find the updated files, follow the link below. If the solution does not work for you, open a new bug report. https://rhn.redhat.com/errata/RHBA-2016-1119.html