On Tue, Feb 23, 2016 at 06:31:59PM -0800, Owen Hofmann wrote: > Specifically, what underlying source of time should be exposed through > kvm-clock and other paravirtual ABIs like the HyperV reference tsc > page? Recently a couple of threads on kvm-list, along with attempts > to produce reliable behavior from kvm-clock on our systems have > highlighted a tension between the current implementation of kvm-clock > and potentially diverging goals for paravirt time. Here are a few: > > 1) kvmclock doesn't work, help?: http://www.spinics.net/lists/kvm/msg125039.html > 2) kvmclock: improve accuracy: http://www.spinics.net/lists/kvm/msg127215.html > 3) KVM-clock: http://www.spinics.net/lists/kvm/msg127774.html > > This question is mostly in regards to kvm-clock in masterclock mode > (with PVCLOCK_TSC_STABLE set). In this mode, is kvm-clock intended to > expose a source of time that is more 'true' than the underlying TSC? > For example, by passing through NTP correction from the host. For the > current implementation, the answer seems to be... why not both? Once > programmed, kvm-clock or the HyperV TSC page will advance with the TSC > multiplied by the frequency specified by kvm. On the other hand, > KVM_GET_CLOCK, KVM_SET_CLOCK, and the Windows reference counter MSR > are measured against corrected time from the host. A guest reading its > pvclock gets a very different result from a host KVM_GET_CLOCK if the > guest has run long enough to for TSC to diverge from NTP time. A VMM > using these ioctls to save and restore clock state can produce wild > time jumps from the guest's perspective. > > The patches in (2) address this mismatch by plumbing updates to clock > frequency through kvm-clock to the guest. This seems like an important > design choice for kvm-clock, and IMO deserves at least a clear > statement of the goals for this interface, if not some more > discussion. Design goals of what interface? KVM_GET_CLOCK / KVM_SET_CLOCK? The interfaces have been introduced to fix a bug. > The (later) thread in (3) claims that synchronizing with > host time is *not* a goal of kvm-clock. It is not. > To me, kvm-clock and the HyperV TSC page are extremely effective as > simply a more enlightened path to the host TSC. Maintaining a > high-performance path to the TSC in the face of updates is tricky - > see the extended comment in pvclock_update_vm_gtod_copy, or the > discussion on the patchset in (2). Is the cost of auditing that the > path from host gettimeofday update -> kvm -> guest pvclock -> guest > gettimeofday both tracks host time correctly and does not produce any > backwards warps worth the added value, if it exists? As an > alternative, implementing KVM_GET_CLOCK or the reference time MSR as a > function of the last update to kvm-clock or the reference TSC page, > respectively, sounds very straightforward. > > (Outside of masterclock mode, the requirement that the client > synchronizes across cpus for montonicity smoothes over a lot of > complexity - periodically updating kvm-clock to the current time is > simple and works.) > > Regardless of my opinion, I think that a clear statement of the design > goals for kvm-clock (and kvm's implementation of the reference TSC > page) would be valuable. Documentation/virtual/kvm/timekeeping.txt -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
