On Wed, Jan 15, 2014 at 12:43:02PM +0100, Paolo Bonzini wrote: > Il 06/01/2014 15:18, Marcelo Tosatti ha scritto: > > > > To fix a problem related to different resolution of TSC and system clock, > > the offset in TSC units is approximated by > > > > delta = vcpu->hv_clock.tsc_timestamp - vcpu->last_guest_tsc > > > > (Guest TSC value at (Guest TSC value at last VM-exit) > > the last kvm_guest_time_update > > call) > > > > Delta is then later scaled using mult,shift pair found in hv_clock > > structure (which is correct against tsc_timestamp in that > > structure). > > > > However, if a frequency change is performed between these two points, > > this delta is measured using different TSC frequencies, but scaled using > > mult,shift pair for one frequency only. > > > > The end result is an incorrect delta. > > > > The bug which this code works around is not the only cause for > > clock backwards events. The global accumulator is still > > necessary, so remove the max_kernel_ns fix and rely on the > > global accumulator for no clock backwards events. > > This is basically reverting commit 1d5f066 (KVM: x86: Fix a possible > backwards warp of kvmclock, 2010-08-19). > > Your commit message basically says that the guest-side 489fb49 (x86, > paravirt: Add a global synchronization point for pvclock, 2010-05-11) is > the real solution to the problem that the host-side commit 1d5f066 was > trying to fix. Right? Yes. > This patch makes vcpu->hv_clock.tsc_timestamp write only. Please > provide a follow up that drops the field entirely, then I'll apply both. OK. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
