8bf00a529967dafbbb210b377c38a15834d1e979 - performance regression?

Michael S. Tsirkin
2013-10-31T00:18:53Z

commit 8bf00a529967dafbbb210b377c38a15834d1e979:
" KVM: VMX: add support for switching of PERF_GLOBAL_CTRL " was
as far as I can tell supposed to bring about performance improvement
on hardware that supports it?
Instead it seems to make the typical case (not running guest
under perf) a bit slower than it used to be.
the cost of VMexit goes up by about 50 cycles
on sandy bridge where the optimization in question
actually is activated.

Why that's not a large regression, it's a far cry from
actually helping performance.

So are we better off not using this feature?
What kind of workload is improved by this change?