Re: What time is it kvm-clock?

["Radim Krčmář" <rkrcmar@redhat.com>]

2016-02-24 17:19-0800, Andy Lutomirski:
> On Wed, Feb 24, 2016 at 3:35 PM, Marcelo Tosatti <mtosatti@redhat.com> wrote:
>> About complaint that "its not well designed whether NTP correction
>> should be applied or not". There are two different things:
>>
>> 1) Host clock and guest clocks synchronized.
>> KVM is not responsible for that, and it can't, because
>> Linux exposes a clock which is created in software
>> and fixed by NTP.
> 
> I don't understand what you mean.
> 
> 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.

kvmclock always is a 1 GHz clock, it just wasn't (maybe still isn't) a
good source of source of 1 GHz.  The guest can't know that it's poor
unless it compares with better 1 GHz sources, for example via NTP.

KVM can't know what 1 GHz is better than the host and vice-versa, so
kvmclock should "accidentally" track CLOCK_BOOTTIME.

But that is related to (2), I think that (1) was mainly about the offset
from CLOCK_REALTIME.

> 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.

Using TSC in the host allows KVM to provide precise CLOCK_REALTIME, but
nothing prevents us from giving host's CLOCK_REALTIME even if the host
is using hpet/... as the source.

>> 2) NTP frequency correction being applied to kvmclock.
>>
>> This only means that the frequency of the pvclock reads
>> in the guest are NTP corrected.
> 
> If the host applied NTP frequency correction to the guest, then I
> would be happy.  Some folks might want this to be optional.

kvmclock provides time in nanoseconds, so I'd argue that NTP corrections
are mandatory.