Re: [RFC 09/10] hw/mos6522: Avoid using discrepant QEMU clock values

[Finn Thain <fthain@linux-m68k.org>]

On Wed, 25 Aug 2021, Mark Cave-Ayland wrote:

> On 24/08/2021 11:09, Finn Thain wrote:
> 
> > mos6522_read() and mos6522_write() may call various functions to determine
> > timer irq state, timer counter value and QEMUTimer deadline. All called
> > functions must use the same value for the present time.
> > 
> > Signed-off-by: Finn Thain <fthain@linux-m68k.org>
> > ---
> >   hw/misc/mos6522.c | 51 +++++++++++++++++++++++++----------------------
> >   1 file changed, 27 insertions(+), 24 deletions(-)
> > 
> > diff --git a/hw/misc/mos6522.c b/hw/misc/mos6522.c
> > index 0dd3ccf945..23a440b64f 100644
> > --- a/hw/misc/mos6522.c
> > +++ b/hw/misc/mos6522.c
> > @@ -39,9 +39,9 @@
> >   /* XXX: implement all timer modes */
> >     static void mos6522_timer1_update(MOS6522State *s, MOS6522Timer *ti,
> > -                                  int64_t current_time);
> > +                                  int64_t now);
> >   static void mos6522_timer2_update(MOS6522State *s, MOS6522Timer *ti,
> > -                                  int64_t current_time);
> > +                                  int64_t now);
> >     static void mos6522_update_irq(MOS6522State *s)
> >   {
> > @@ -52,12 +52,12 @@ static void mos6522_update_irq(MOS6522State *s)
> >       }
> >   }
> >   -static unsigned int get_counter(MOS6522State *s, MOS6522Timer *ti)
> > +static unsigned int get_counter(MOS6522State *s, MOS6522Timer *ti, int64_t
> > now)
> >   {
> >       int64_t d;
> >       unsigned int counter;
> >   -    d = muldiv64(qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) - ti->load_time,
> > +    d = muldiv64(now - ti->load_time,
> >                    ti->frequency, NANOSECONDS_PER_SECOND);
> >         if (ti->index == 0) {
> > @@ -89,7 +89,7 @@ static void set_counter(MOS6522State *s, MOS6522Timer *ti,
> > unsigned int val)
> >   }
> >     static int64_t get_next_irq_time(MOS6522State *s, MOS6522Timer *ti,
> > -                                 int64_t current_time)
> > +                                 int64_t now)
> >   {
> >       int64_t d, next_time;
> >       unsigned int counter;
> > @@ -99,7 +99,7 @@ static int64_t get_next_irq_time(MOS6522State *s,
> > MOS6522Timer *ti,
> >       }
> >         /* current counter value */
> > -    d = muldiv64(qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) - ti->load_time,
> > +    d = muldiv64(now - ti->load_time,
> >                    ti->frequency, NANOSECONDS_PER_SECOND);
> >         /* the timer goes down from latch to -1 (period of latch + 2) */
> > @@ -123,20 +123,19 @@ static int64_t get_next_irq_time(MOS6522State *s,
> > MOS6522Timer *ti,
> >       trace_mos6522_get_next_irq_time(ti->latch, d, next_time - d);
> >       next_time = muldiv64(next_time, NANOSECONDS_PER_SECOND, ti->frequency)
> > +
> >                            ti->load_time;
> > -
> > -    if (next_time <= current_time) {
> > -        next_time = current_time + 1;
> > -    }
> >       return next_time;
> >   }
> >     static void mos6522_timer1_update(MOS6522State *s, MOS6522Timer *ti,
> > -                                 int64_t current_time)
> > +                                  int64_t now)
> >   {
> >       if (!ti->timer) {
> >           return;
> >       }
> > -    ti->next_irq_time = get_next_irq_time(s, ti, current_time);
> > +    ti->next_irq_time = get_next_irq_time(s, ti, now);
> > +    if (ti->next_irq_time <= now) {
> > +        ti->next_irq_time = now + 1;
> > +    }
> >       if ((s->ier & T1_INT) == 0 ||
> >           ((s->acr & T1MODE) == T1MODE_ONESHOT && ti->oneshot_fired)) {
> >           timer_del(ti->timer);
> > @@ -146,12 +145,15 @@ static void mos6522_timer1_update(MOS6522State *s,
> > MOS6522Timer *ti,
> >   }
> >     static void mos6522_timer2_update(MOS6522State *s, MOS6522Timer *ti,
> > -                                 int64_t current_time)
> > +                                  int64_t now)
> >   {
> >       if (!ti->timer) {
> >           return;
> >       }
> > -    ti->next_irq_time = get_next_irq_time(s, ti, current_time);
> > +    ti->next_irq_time = get_next_irq_time(s, ti, now);
> > +    if (ti->next_irq_time <= now) {
> > +        ti->next_irq_time = now + 1;
> > +    }
> >       if ((s->ier & T2_INT) == 0 || (s->acr & T2MODE) || ti->oneshot_fired)
> > {
> >           timer_del(ti->timer);
> >       } else {
> > @@ -163,9 +165,10 @@ static void mos6522_timer1_expired(void *opaque)
> >   {
> >       MOS6522State *s = opaque;
> >       MOS6522Timer *ti = &s->timers[0];
> > +    int64_t now = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
> >         ti->oneshot_fired = true;
> > -    mos6522_timer1_update(s, ti, ti->next_irq_time);
> > +    mos6522_timer1_update(s, ti, now);
> 
> Presumably using ti->next_irq_time has already fixed the current time to 
> be that at which the timer routine actually expired, rather than the 
> current executing time. Are you seeing large differences in these 
> numbers that can cause timer drift? If so, I'm wondering if this change 
> should be in a separate patch.
> 

You're right. This change has more relevance to the synchronization work 
in the following patch. It's not really covered by the commit log here.

> >       s->ifr |= T1_INT;
> >       mos6522_update_irq(s);
> >   }
> > @@ -174,9 +177,10 @@ static void mos6522_timer2_expired(void *opaque)
> >   {
> >       MOS6522State *s = opaque;
> >       MOS6522Timer *ti = &s->timers[1];
> > +    int64_t now = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
> >         ti->oneshot_fired = true;
> > -    mos6522_timer2_update(s, ti, ti->next_irq_time);
> > +    mos6522_timer2_update(s, ti, now);
> 
> And same again here.
> 

I'll find a better way to split up these patches.

> >       s->ifr |= T2_INT;
> >       mos6522_update_irq(s);
> >   }
> > @@ -233,12 +237,12 @@ uint64_t mos6522_read(void *opaque, hwaddr addr,
> > unsigned size)
> >           val = s->dira;
> >           break;
> >       case VIA_REG_T1CL:
> > -        val = get_counter(s, &s->timers[0]) & 0xff;
> > +        val = get_counter(s, &s->timers[0], now) & 0xff;
> >           s->ifr &= ~T1_INT;
> >           mos6522_update_irq(s);
> >           break;
> >       case VIA_REG_T1CH:
> > -        val = get_counter(s, &s->timers[0]) >> 8;
> > +        val = get_counter(s, &s->timers[0], now) >> 8;
> >           break;
> >       case VIA_REG_T1LL:
> >           val = s->timers[0].latch & 0xff;
> > @@ -247,12 +251,12 @@ uint64_t mos6522_read(void *opaque, hwaddr addr,
> > unsigned size)
> >           val = (s->timers[0].latch >> 8) & 0xff;
> >           break;
> >       case VIA_REG_T2CL:
> > -        val = get_counter(s, &s->timers[1]) & 0xff;
> > +        val = get_counter(s, &s->timers[1], now) & 0xff;
> >           s->ifr &= ~T2_INT;
> >           mos6522_update_irq(s);
> >           break;
> >       case VIA_REG_T2CH:
> > -        val = get_counter(s, &s->timers[1]) >> 8;
> > +        val = get_counter(s, &s->timers[1], now) >> 8;
> >           break;
> >       case VIA_REG_SR:
> >           val = s->sr;
> > @@ -360,10 +364,9 @@ void mos6522_write(void *opaque, hwaddr addr, uint64_t
> > val, unsigned size)
> >           }
> >           mos6522_update_irq(s);
> >           /* if IER is modified starts needed timers */
> > -        mos6522_timer1_update(s, &s->timers[0],
> > -                              qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL));
> > -        mos6522_timer2_update(s, &s->timers[1],
> > -                              qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL));
> > +        now = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
> > +        mos6522_timer1_update(s, &s->timers[0], now);
> > +        mos6522_timer2_update(s, &s->timers[1], now);
> >           break;
> >       default:
> >           g_assert_not_reached();
> 
> In terms of functionality it shouldn't really matter (since you have a 
> ns clock compared with a timer that can manage small frequencies in 
> comparison) but I can see how having a constant clock time throughout 
> the entire calculation process could be useful for debugging.
> 

I found it impossible to reason about program behaviour with so many calls 
to qemu_clock_get_ns().

Thanks for your review.