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

[Maxim Levitsky]

On Mon, 2020-12-07 at 10:04 -0800, Andy Lutomirski wrote:
> > On Dec 7, 2020, at 9:00 AM, Maxim Levitsky <mlevitsk@xxxxxxxxxx> wrote:
> > 
> > On Mon, 2020-12-07 at 08:53 -0800, Andy Lutomirski wrote:
> > > > > On Dec 7, 2020, at 8:38 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> > > > 
> > > > On Mon, Dec 07 2020 at 14:16, Maxim Levitsky wrote:
> > > > > > On Sun, 2020-12-06 at 17:19 +0100, Thomas Gleixner wrote:
> > > > > > From a timekeeping POV and the guests expectation of TSC this is
> > > > > > fundamentally wrong:
> > > > > > 
> > > > > >     tscguest = scaled(hosttsc) + offset
> > > > > > 
> > > > > > The TSC has to be viewed systemwide and not per CPU. It's systemwide
> > > > > > used for timekeeping and for that to work it has to be synchronized. 
> > > > > > 
> > > > > > Why would this be different on virt? Just because it's virt or what? 
> > > > > > 
> > > > > > Migration is a guest wide thing and you're not migrating single vCPUs.
> > > > > > 
> > > > > > This hackery just papers over he underlying design fail that KVM looks
> > > > > > at the TSC per vCPU which is the root cause and that needs to be fixed.
> > > > > 
> > > > > I don't disagree with you.
> > > > > As far as I know the main reasons that kvm tracks TSC per guest are
> > > > > 
> > > > > 1. cases when host tsc is not stable 
> > > > > (hopefully rare now, and I don't mind making
> > > > > the new API just refuse to work when this is detected, and revert to old way
> > > > > of doing things).
> > > > 
> > > > That's a trainwreck to begin with and I really would just not support it
> > > > for anything new which aims to be more precise and correct.  TSC has
> > > > become pretty reliable over the years.
> > > > 
> > > > > 2. (theoretical) ability of the guest to introduce per core tsc offfset
> > > > > by either using TSC_ADJUST (for which I got recently an idea to stop
> > > > > advertising this feature to the guest), or writing TSC directly which
> > > > > is allowed by Intel's PRM:
> > > > 
> > > > For anything halfways modern the write to TSC is reflected in TSC_ADJUST
> > > > which means you get the precise offset.
> > > > 
> > > > The general principle still applies from a system POV.
> > > > 
> > > >    TSC base (systemwide view) - The sane case
> > > > 
> > > >    TSC CPU  = TSC base + TSC_ADJUST
> > > > 
> > > > The guest TSC base is a per guest constant offset to the host TSC.
> > > > 
> > > >    TSC guest base = TSC host base + guest base offset
> > > > 
> > > > If the guest want's this different per vCPU by writing to the MSR or to
> > > > TSC_ADJUST then you still can have a per vCPU offset in TSC_ADJUST which
> > > > is the offset to the TSC base of the guest.
> > > 
> > > How about, if the guest wants to write TSC_ADJUST, it can turn off all paravirt features and keep both pieces?
> > > 
> > 
> > This is one of the things I had in mind recently.
> > 
> > Even better, we can stop advertising TSC_ADJUST in CPUID to the guest 
> > and forbid it from writing it at all.
> 
> Seems reasonable to me.
> 
> It also seems okay for some MSRs to stop working after the guest enabled new PV timekeeping.
This is a very good idea!

> 
> I do have a feature request, though: IMO it would be quite nifty if the new kvmclock structure could also expose NTP corrections. In other words, if you could expose enough info to calculate CLOCK_MONOTONIC_RAW, CLOCK_MONOTONIC, and CLOCK_REALTIME, then we could have paravirt NTP.
> 
> Bonus points if whatever you do for CLOCK_REALTIME also exposes leap seconds in a race free way :). But I suppose that just exposing TAI and letting the guest deal with the TAI - UTC offset itself would get the job done just fine.

This is a good idea too.
As I understand it, this gives a justification to a new kvmclock purpose,
which wouldn't be focused anymore on correcting the tsc shortcomings
(unstable/unscalable tsc), but more on things like that.
I like that idea.

Best regards,
	Maxim Levitsky