On (21/06/07 15:54), Maxim Levitsky wrote: [..] > Also I would like to add my .02 cents on my observations on what happens when I suspend my system > with guests running, which I do once in a while. > I haven't dug deep into it yet as host suspend with VM running wasn't high on my priority list. > > First of all after a host suspend/resume cycle (and that is true on all 3 machines I own), > the host TSC is reset to 0 on all CPUs, thus while it is still synchronized, it jumps backward. > > Host kernel has no issues coping with this. > > Guests however complain about clocksource watchdog and mark the tsc clocksource as unstable, > at least when invtsc is used (regardless of this patch, I wasn't able to notice a difference > with and without it yet). > > > [ 287.515864] clocksource: timekeeping watchdog on CPU0: Marking clocksource 'tsc' as unstable because the skew is too large: > [ 287.516926] clocksource: 'kvm-clock' wd_now: 4437767926 wd_last: 429c3c42f5 mask: ffffffffffffffff > [ 287.527100] clocksource: 'tsc' cs_now: c33f6ce157 cs_last: c1be2ad19f mask: ffffffffffffffff > [ 287.528493] tsc: Marking TSC unstable due to clocksource watchdog > [ 287.556640] clocksource: Switched to clocksource kvm-clock > > > This is from Intel system with stable TSC, but I have seen this on my AMD systems as well, > but these have other issues which might affect this (see below). > > AFAIK, we have code in kvm_arch_hardware_enable for this exact case but it might not work > correctly or be not enough to deal with this. > > Also I notice that this code sets kvm->arch.backwards_tsc_observed = true which > in turn disables the master clock which is not good as well. > > I haven't yet allocated time to investigate this. > > > Another bit of information which I didn't start a discussion (but I think I should), > which is relevant to AMD systems, is in 'unsynchronized_tsc' function. > > On AMD guest it will mark the TSC as unstable in the guest as long as invtsc is not used. > I patched that code out for myself, that is why I am mentioning it. If you are going to fork this discussion the could you please Cc Suleiman and me? I believe we are having a bunch of problems with the guest clocks here on our side.
