KVM: x86: fix tsc catchup issue with tsc scaling
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. Signed-off-by: NMarcelo Tosatti <mtosatti@redhat.com> Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
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