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

[Marcelo Tosatti <mtosatti@redhat.com>]

On Mon, Dec 07, 2020 at 10:04:45AM -0800, Andy Lutomirski wrote:
> 
> > On Dec 7, 2020, at 9:00 AM, Maxim Levitsky <mlevitsk@redhat.com> wrote:
> > 
> > On Mon, 2020-12-07 at 08:53 -0800, Andy Lutomirski wrote:
> >>>> On Dec 7, 2020, at 8:38 AM, Thomas Gleixner <tglx@linutronix.de> 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.
> 
> 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.

Hi Andy,

Any reason why drivers/ptp/ptp_kvm.c does not work for you?

> 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.