The implementation in QEMU that I've settled upon makes do with just
COUNT_CTL and COUNT_RESUME, but with a slight kernel modification so
that COUNT_RESUME is writeable (to any positive monotonic nanosecond
value <= now). It works fairly cleanly and correctly even with stopping
and starting VM clock (gdb, stop/cont, savevm/loadvm, live migration),
to match the behaviour of the existing mips cpu timer emulation, so I
plan to drop this bias patch, and will post a v2 patchset soon with just
a few modifications.