> 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. 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.
