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

[Marcelo Tosatti]

On Tue, Dec 08, 2020 at 06:25:13PM +0200, Maxim Levitsky wrote:
> On Tue, 2020-12-08 at 17:02 +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?
> > 
> > > 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.
> 
> The "bug" is that if VMM moves a hardware time counter (tsc or anything else) 
> forward by large enough value in one go, 
> then the guest kernel will supposingly have an overflow in the time code.
> I don't consider this to be a buggy VMM behavior, but rather a kernel
> bug that should be fixed (if this bug actually exists)

It exists.

> Purely in theory this can even happen on real hardware if for example SMM handler
> blocks a CPU from running for a long duration, or hardware debugging
> interface does, or some other hardware transparent sleep mechanism kicks in
> and blocks a CPU from running.
> (We do handle this gracefully for S3/S4)
> 
> > 
> > Aside of that I think I made it pretty clear what the right thing to do
> > is.
> 
> This is orthogonal to this issue of the 'bug'. 
> Here we are not talking about per-vcpu TSC offsets, something that I said 
> that I do agree with you that it would be very nice to get rid of.
>  
> We are talking about the fact that TSC can jump forward by arbitrary large
> value if the migration took arbitrary amount of time, which 
> (assuming that the bug is real) can crash the guest kernel.

QE reproduced it.

> This will happen even if we use per VM global tsc offset.
> 
> So what do you think?
> 
> Best regards,
> 	Maxim Levitsky
> 
> > 
> > Thanks,
> > 
> >         tglx
> > 
>