Re: KVM: x86: fix tsc catchup issue with tsc scaling

[Marcelo Tosatti <mtosatti@redhat.com>]

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.