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
