Re: What time is it kvm-clock?

[Marcelo Tosatti <mtosatti@redhat.com>]

On Fri, Feb 26, 2016 at 09:02:16AM -0800, Andy Lutomirski wrote:
> On Thu, Feb 25, 2016 at 4:20 AM, Radim Krčmář <rkrcmar@redhat.com> wrote:
> > 2016-02-24 19:50-0800, Owen Hofmann:
> >> On Wed, Feb 24, 2016 at 5:19 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> >>> Of course the guest can run its own NTP daemon or similar adjtimex
> >>> caller and cause the guest to stop tracking the host.  But if the host
> >>> passed CLOCK_MONOTONIC through, then the guest would, by default,
> >>> treat kvm-clock as an exactly 1GHz source and would then expose a
> >>> disciplined NTP-tracking CLOCK_MONOTONIC through to its user apps even
> >>> without an NTP client on the guest.
> >>>
> >>> If integration with the POSIX clock core were provided, the guest
> >>> would learn to consume the host's CLOCK_REALTIME as well, as long as
> >>> the host uses the tsc as its clocksource.
> >>
> >> Your proposal, which I'd describe as a direct passthrough (to the
> >> extent possible) of the host gettimeofday vdso to a kvm guest, sounds
> >> like a much better way to get clock frequency adjustments from the
> >> host to the guest. But I don't know if I can think of a reason to do
> >> this besides "hey you don't have to run ntp". Is there a situation you
> >> have in mind that this helps out?
> >
> > Running NTP only on the host is a good reason.
> > (And probably the only reason I'd call good, because any software that
> >  passes TSC or CLOCK_MONOTONIC timestamps between hosts needs to handle
> >  their differences.)
> 
> There are handful of distributed algorithms that benefit from clocks
> with a bounded worst-case synchronization error.  I think that Google
> uses some.  If some cloud provider were to provide, say, 10ms max
> CLOCK_REALTIME error and pass CLOCK_REALTIME through using kvm-clock,
> it could be quite useful.
> 
> --Andy

Why would you want to do that again?
To fix the suspend/resume problem?