On 9/23/19 5:08 PM, Peter Maydell wrote: > On Mon, 23 Sep 2019 at 15:54, Philippe Mathieu-Daudé <phi...@redhat.com> > wrote: >> >> On 9/23/19 4:40 PM, Peter Maydell wrote: >>> On Sat, 21 Sep 2019 at 11:17, Philippe Mathieu-Daudé <phi...@redhat.com> >>> 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.)
OK I'll improve the comment. > 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. Yes :/ Thanks! Phil.
