On Wed, Feb 24, 2016 at 6:14 AM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote: > > > On 24/02/2016 03:31, 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. > > Right, in fact that's why QEMU is not really using KVM_GET_CLOCK > anymore. In retrospect, the "fix" in QEMU was probably a bad idea. It > would have been better to fix KVM_GET_CLOCK. > >> 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. > > Yes, we could do that too. > > I think that vgettsc and do_monotonic_boot also would have to use the > TSC frequency instead the NTP-adjusted host clock. > >> (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. > > Since we cannot change the past, having kvmclock synchronize with the > host TSC frequency is the only choice we can make. > Could we introduce a new kvm-clock or perhaps opt-in mode that: a) uses hypervisor-supplied IO pages and, b) synchronizes to host CLOCK_MONOTONIC instead of some bizarre non-suspend-resume-safe not-really-well-defined hybrid? --Andy -- 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
