The availability of hardware counters in computers is essential both to the applications in charge of timekeeping, and those in need of accurate timestamping. Newer counters are now supported by open source operating systems, but the access interfaces are unnecessarily restricted, and in particular fail to satisfy the needs of feed-forward based synchronization algorithms. In this paper we present modifications to the Linux and FreeBSD kernels to enable any application to access all available counters in an unrestricted way, and then evaluate their stability, latency and robustness to stress. We demonstrate how the feed-forward based RADclock can, through this interface, make use of any of several counters, and achieve the same microsecond synchronization with each.