Re: [PATCH] hw/ptimer: Assert next_event is newer than last_event

[Peter Maydell]

On Mon, 23 Sep 2019 at 15:54, Philippe Mathieu-Daudé <address@hidden> wrote:
>
> On 9/23/19 4:40 PM, Peter Maydell wrote:
> > On Sat, 21 Sep 2019 at 11:17, Philippe Mathieu-Daudé <address@hidden> wrote:
> >>
> >> If the period is too big, the 'delta * period' product result
> >> might overflow, resulting in a negative number, then the
> >> next_event ends before the last_event. This is buggy, as there
> >> is no forward progress. Assert this can not happen.

> > Can this only happen if a QEMU timer model using the ptimer
> > code has a bug, or is it guest-triggerable for some of our
> > timer models?
>
> I hit this running a raspi4 guest, I had incorrectly initialized a clock
> using the core cpu frequency, while I had to use the APB one (in my
> case, core_cpu_freq / 2). The guest use a high value to configure a slow
> timer, which in my buggy case made QEMU hang in hard way to debug.
>
> So yes, it seems guest-triggerable if the implementation is broken.
> Using assert() is OK for broken implementation, right?

Yeah, if this can only happen if QEMU code is broken then
an assert is OK. I was just trying to find out what the
cause was, since "this is buggy" isn't specific about where
the bug is.

> Or should we audit all ptimer calls?

I don't think we specifically need an audit. We could perhaps
expand the comment by the assert to specifically say that if
the calculation of the next event overflowed then this indicates
a bug in the QEMU device model using the ptimer API, so if
somebody else runs into the assert they have a hint about
what to look at. (An overflowed next_event indicates a time
incredibly far in the future, given that it's a nanosecond
time in an int64_t.)

The other approach I thought of would be to make the ptimer
code handle this sort of after-the-end-of-QEMU-universe time
by saturating next_event to INT64_MAX rather than letting it
overflow and wrap. Unfortunately while this would be fine for
the 'timer event' part of the code, it would break
ptimer_get_count() which calculates the current counter
value by looking at the difference between the current
time and the time of the next event (fixable but only with
a bunch of messing about to treat a next_event of INT64_MAX
as equivalent to the counter being disabled and tracking
the counter value in s->delta). So an assert is the
best thing I think.

thanks
-- PMM