Re: [Qemu-devel] [PATCH v3 5/5] hpet 'driftfix': add code in hpet_timer() to compensate delayed callbacks and coalesced interrupts

[Ulrich Obergfell]

Hi Marcelo,
 
> Other than that, shouldnt reset accounting variables to init state on
> write to GLOBAL_ENABLE_CFG / writes to main counter?

I'd suggest to initialize/reset the driftfix-related fields in the
'HPETTimer' structure (including the backlog of unaccounted ticks)
in the following situations.


- When the guest o/s sets the 'CFG_ENABLE' bit (overall enable) in
  the General Configuration Register.

  This is the place in hpet_ram_writel():

        case HPET_CFG:
            ...
            if (activating_bit(old_val, new_val, HPET_CFG_ENABLE)) {
                ...
                for (i = 0; i < s->num_timers; i++) {
                    if ((&s->timer[i])->cmp != ~0ULL) {
                        // initialize / reset fields here
                        hpet_set_timer(&s->timer[i]);


- When the guest o/s sets the 'TN_ENABLE' bit (timer N interrupt
  enable) in the Timer N Configuration and Capabilities Register.

  This is the place in hpet_ram_writel():

        case HPET_TN_CFG:
            ...
            if (activating_bit(old_val, new_val, HPET_TN_ENABLE)) {
                // initialize / reset fields here
                hpet_set_timer(timer);


This should cover cases such as ...

- first time initialization of HPET & timers during guest o/s boot.
  (includes kexec and reboot)

- if a guest o/s stops and restarts the timer.
  (for example, to change the comparator register value or
   to switch a timer between periodic mode and non-periodic mode)


Regards,

Uli