On Mon, Dec 07, 2020 at 10:04:45AM -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. > > 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.
