Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE

[Marcelo Tosatti]

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...