Re: TSC deadline timer in guests vs. migration?

[Marcelo Tosatti <mtosatti@redhat.com>]

On Tue, Jul 05, 2016 at 03:34:20PM +0200, Paolo Bonzini wrote:
> 
> 
> On 05/07/2016 15:04, Eduardo Habkost wrote:
> > On Tue, Jul 05, 2016 at 09:51:41AM -0300, Marcelo Tosatti wrote:
> >> On Mon, Jul 04, 2016 at 04:45:06PM -0300, Eduardo Habkost wrote:
> >>> On Mon, Jul 04, 2016 at 04:30:08PM -0300, Marcelo Tosatti wrote:
> >>>> On Mon, Jul 04, 2016 at 01:01:42PM +0200, Paolo Bonzini wrote:
> >>>>> Can bad things happen if a guest using the TSC deadline timer is
> >>>>> migrated?  The guest doesn't re-calibrate the TSC after migration, and
> >>>>> the TSC frequency can and will change unless your data center is
> >>>>> perfectly homogeneous.
> >>>>
> >>>> It can fire earlier if the destination runs at a higher frequency.
> >>>> It will fire past the configured time if the destination runs at a slower frequency.
> >>>>
> >>>> Suppose the first case is worse.
> >>>>
> >>>> Should convert the expiration time to nanoseconds i suppose, and then 
> >>>> convert back on the destination.
> >>>
> >>> This won't make any difference if the guest sets up a new timer
> >>> after migration (but using the old TSC frequency), will it?
> >>
> >> It does, because the timer setup traps to the host, where you can
> >> convert it to the proper value:
> > 
> > To convert it to the proper value, you need to know what's the
> > TSC frequency the guest is assuming. How would you do that?
> 
> In theory with KVM_SET_TSC_KHZ, but it has to be specified manually and

Just read the TSC value where the guest has booted and migrate that.

> I'm not sure anyone has tested this recently.  I'm also not sure how
> robust it is, but I'm sure it's fairly slow because it triggers a
> kvmclock update on every vmentry.

But agreed on that the rest of TSC scaling is poorly thought through.

> tsc_always_catchup is high on the list of things that I hate in KVM.

It can be dropped probably. I don't see a need for it (tsc scaling 
was never supported properly because of kvmclock).

> I'd like to make KVM_SET_TSC_KHZ fail if TSC scaling is not supported by
> the processor.
> 
> Paolo

Thats a good idea.