On 2011-08-22 22:36, Anthony Liguori wrote: > On 08/22/2011 03:28 PM, Jan Kiszka wrote: >> On 2011-08-22 21:21, Anthony Liguori wrote: >>> This replaces all of the QEMU timer code with GHRTimer, dramatically >>> simplifying >>> time keeping in QEMU while making it possible to use QEMUTimer code >>> outside of >>> the main loop. The later is critical to building unit tests. >>> >>> This is an RFC because I'm sure this breaks things as it changes >>> things. QEMU >>> time keeping is quite a mess today. Here's what we do today: >>> >>> 1) We have three clocks: >>> a) the real time clock, based on system time, not monotonic >>> b) the host clock, based on the real time clock, monotonic by >>> detecting >>> movements backward in time >>> c) the vm clock, based on real time clock but may start/stop with >>> the guest >> >> Not quite correct. We have: >> >> - QEMU_CLOCK_REALTIME: Based on monotonic source *if* the host >> supports it (there were probably once some stone-old Linuxes or >> BSDs), otherwise based on gettimeofday, i.e. non-monotonic. Always >> monotonic on Windows. > > The only clock on Linux that is truly monotonic is CLOCK_MONOTONIC_RAW > which is very new (2.6.28+). CLOCK_MONOTONIC is not actually monotonic > as it's subject to adjustments.
CLOCK_MONOTONIC may be subject to frequency tuning while CLOCK_MONOTONIC_RAW is not. That does not and must not (for POSIX compliance) make the former non-monotonic. > >> - QEMU_CLOCK_VIRTUAL: Without -icount, same as above, but stops when >> the guest is stopped. The offset to compensate for stopped >> times is based on TSC, not sure why. With -icount, things get more >> complicated, Paolo had some nice explanations for the details. >> >> - QEMU_CLOCK_HOST: That's the one always based on the host's system >> time (CLOCK_REALTIME) >> + it takes potentially configured offsets into acount >> + users of that clock can register callbacks on time warps into the >> past (to adjust pending timers) > > Right, my assertion is that time warps are a bug as far as QEMU is > concerned. Is there any reason why the guest should care at all about > time warping in the host? Yes, to synchronize on an accurate host clock source without the need to add NTP-like services (or kvmclock) to the guest. So the warps are conceptionally valid, though still not very welcome. > >>> 2) A "cpu ticks" clock that uses platform specific mechanisms (inline >>> asm) >>> >>> 3) Various clock source implementations that may use a periodic timer >>> or a >>> a dynamic time source. We have different implementations for >>> different >>> platforms >>> >>> 4) Time events are delivered via SIGALRM which means we end up >>> getting EINTRs >>> very often in QEMU. This is fairly annoying. Signals also race >>> against >>> select leading to a very ugly set of work arounds involving >>> writing data to >>> pipes. This is the sort of stuff in Unix programming that I wish >>> I never had >>> to learn about and am very eager to eliminate in QEMU :-) >>> >>> (2) is just plain broken. In modern operating systems, >>> gettimeofday() is >>> optimized CPU instructions when they can be used safely. Often they >>> can't be >>> used safely and we ignore that in QEMU. For instance, on x86, RDTSC >>> races with >>> the scheduler (not to mention that the TSC is infamously unstable >>> across cores). >>> The kernel does the right thing here and provides the fastest method >>> that's >>> correct. >> >> I basically agree. Likely, these optimizations date back to the days >> Linux had no fast gettimeofday syscalls. Not sure what the state on >> other UNIXes is, but it's likely not worth keeping these optimizations. >> Let's drop that one first and separately. >> >>> >>> (1.a) seems like a bug more than a feature. I don't see a lot of >>> disadvantages >>> to using a monotonic time source. >>> >>> (1.b) is a bit naive in its current form. Modern kernels export a truly >>> monotonic time source which has a reliable frequency. Even though >>> (1.b) detects >>> backwards jumps, it doesn't do anything about large forward jumps >>> which can also >>> be problematic. >> >> These two assessments are partly just wrong, partly fail to see the real >> use case. QEMU_CLOCK_HOST serves the very valid scenarios where a guest >> clock shall be kept synchronized on the host time, also following its >> jumps accordingly without stalling timers. > > The only reason we see jumps at all is because we're using > CLOCK_MONOTONIC or CLOCK_REALTIME. If we used CLOCK_MONOTONIC_RAW, we > don't see any jumps at all. CLOCK_MONOTONIC will not jump backward as well, so is perfectly fine and better portable. Backward jumps cannot be avoided when using a host system clock that is subject to follow a more accurate external source. But having such source for RTC emulation e.g. is a useful feature. Jan
signature.asc
Description: OpenPGP digital signature
