The performance counter values for VF part of the pipeline (IA_VERTICES_COUNT, IA_PRIMITIVES_COUNT) are always 0. Gathering of these counters should be explicitly enabled in VF_STATE.
According to the specs, the same should apply to *_INVOCATION_COUNT counters, but those come out fine.
As a workaround, INTEL_DEBUG=stats enables the gathering.
Should be fixed by:
Author: Ben Widawsky <email@example.com>
Date: Wed Nov 12 18:49:16 2014 -0800
i965: Always enable VF statistics
Tested, indeed fixed.