I've reimplemented this idea multiple times and have finally settled on the following.

The idea is simple: Replace isrdmsr with a bit vector that has flags for reading, writing, and polling, along with a set of flags for when to read MPERF (at the start of the op, at the end, and before each polled read).

This feature is the best way to get a handle on how long it takes for certain MSRs to be updated, e.g., energy and temperature. It can also be useful for implementing what is effectively a userspace signal: the op completes (and the ioctl() returns) when the value of the given MSR changes.