On Tue, Dec 08, 2020 at 05:02:07PM +0100, Thomas Gleixner wrote: > On Tue, Dec 08 2020 at 16:50, Maxim Levitsky wrote: > > On Mon, 2020-12-07 at 20:29 -0300, Marcelo Tosatti wrote: > >> > +This ioctl allows to reconstruct the guest's IA32_TSC and TSC_ADJUST value > >> > +from the state obtained in the past by KVM_GET_TSC_STATE on the same vCPU. > >> > + > >> > +If 'KVM_TSC_STATE_TIMESTAMP_VALID' is set in flags, > >> > +KVM will adjust the guest TSC value by the time that passed since the moment > >> > +CLOCK_REALTIME timestamp was saved in the struct and current value of > >> > +CLOCK_REALTIME, and set the guest's TSC to the new value. > >> > >> This introduces the wraparound bug in Linux timekeeping, doesnt it? > > Which bug? max_cycles overflow. Sent a message to Maxim describing it. > > > It does. > > Could you prepare a reproducer for this bug so I get a better idea about > > what are you talking about? > > > > I assume you need very long (like days worth) jump to trigger this bug > > and for such case we can either work around it in qemu / kernel > > or fix it in the guest kernel and I strongly prefer the latter. > > > > Thomas, what do you think about it? > > For one I have no idea which bug you are talking about and if the bug is > caused by the VMM then why would you "fix" it in the guest kernel. 1) Stop guest, save TSC value of cpu-0 = V. 2) Wait for some amount of time = W. 3) Start guest, load TSC value with V+W. Can cause an overflow on Linux timekeeping. > Aside of that I think I made it pretty clear what the right thing to do > is. Sure: the notion of a "unique TSC offset" already exists (it is detected by write TSC logic, and not explicit in the interface, though). But AFAIK it works pretty well. Exposing a single TSC value on the interface level seems alright to me...
