|
From: | Anthony Liguori |
Subject: | Re: [Qemu-devel] [PATCH 2/2] [RFC] time: refactor QEMU timer to use GHRTimer |
Date: | Mon, 22 Aug 2011 16:55:25 -0500 |
User-agent: | Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.17) Gecko/20110516 Lightning/1.0b2 Thunderbird/3.1.10 |
On 08/22/2011 03:49 PM, Jan Kiszka wrote:
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 guestNot 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.
Yes, you are right. I got myself confused.
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.
I think its of limited utility. The RTC isn't universally used for time keeping. There's also no guarantee that the guest isn't going to be upset by this.
I think a better approach is to simply have a verb in qemu-ga to set/get the guest time. That let's you implement clock adjustment without having to worry about NTP. I'm happy to add that as part of this series.
I don't think messing around with this stuff belongs in the QEMU clock layer though.
Regards, Anthony Liguori
Jan
[Prev in Thread] | Current Thread | [Next in Thread] |