Messages in this thread | | | Date | Wed, 9 Nov 2016 19:14:07 -0200 | From | Marcelo Tosatti <> | Subject | Re: [PATCH] kvm: kvmclock: let KVM_GET_CLOCK return whether the master clock is in use |
| |
On Wed, Nov 09, 2016 at 06:17:16PM -0200, Marcelo Tosatti wrote: > On Wed, Nov 09, 2016 at 06:12:50PM -0200, Marcelo Tosatti wrote: > > On Wed, Nov 09, 2016 at 05:48:15PM +0100, Paolo Bonzini wrote: > > > Userspace can read the exact value of kvmclock by reading the TSC > > > and fetching the timekeeping parameters out of guest memory. This > > > however is brittle and not necessary anymore with KVM 4.11. > > > > Hi Paolo, > > > > Can you point to commit or explanation why that is not the case anymore? > > Thanks > > I don't see how thats possible given the: > > * TSC > * TSC (timer interrupt + TSC deltas) (AKA host TSC clocksource and > * CLOCK_MONOTONIC). > > Clocks currently drift from each other (therefore are not monotonic). > > Have you confirmed CLOCK_MONOTONIC_RAW and TSC are monotonic?
I think the benefits of CLOCK_MONOTONIC_RAW (which supposedly is monotonic with TSC for large lenghts of time) outweight the downsides (just have to check that).
The downsides are that on migration from
source: TSC with freq Y. dest: TSC with freq X.
Suddenly kvmclock starts drifting at a different rate, unsure how well Windows time service and older NTPd respond to this (might be a regression).
| |