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

[Thomas Gleixner]

On Tue, Dec 08 2020 at 15:11, Marcelo Tosatti wrote:
> 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.

Truly helpful. Why the hell did you not talk to me when you ran into
that the first time?

>> 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.

Yes, because you violate the basic assumption which Linux timekeeping
makes. See the other mail in this thread.

Thanks,

        tglx