hw/timer/nrf51_timer: Don't lose time when timer is queried in tight loop

hw/timer/nrf51_timer: Don't lose time when timer is queried in tight loop

The nrf51_timer has a free-running counter which we implement using
the pattern of using two fields (update_counter_ns, counter) to track
the last point at which we calculated the counter value, and the
counter value at that time.  Then we can find the current counter
value by converting the difference in wall-clock time between then
and now to a tick count that we need to add to the counter value.

Unfortunately the nrf51_timer's implementation of this has a bug
which means it loses time every time update_counter() is called.
After updating s->counter it always sets s->update_counter_ns to
'now', even though the actual point when s->counter hit the new value
will be some point in the past (half a tick, say).  In the worst case
(guest code in a tight loop reading the counter, icount mode) the
counter is continually queried less than a tick after it was last
read, so s->counter never advances but s->update_counter_ns does, and
the guest never makes forward progress.

The fix for this is to only advance update_counter_ns to the
timestamp of the last tick, not all the way to 'now'.  (This is the
pattern used in hw/misc/mps2-fpgaio.c's counter.)

Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Message-id: 20230606134917.3782215-1-peter.maydell@linaro.org