Re: [RFC 2/2] x86/kvmclock: Use host timekeeping.

[Suleiman Souhlal <suleiman@google.com>]

On Fri, Sep 20, 2019 at 10:33 PM Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>
> Suleiman Souhlal <suleiman@google.com> writes:
>
> > When CONFIG_KVMCLOCK_HOST_TIMEKEEPING is enabled, and the host
> > supports it, update our timekeeping parameters to be the same as
> > the host. This lets us have our time synchronized with the host's,
> > even in the presence of host NTP or suspend.
> >
> > When enabled, kvmclock uses raw tsc instead of pvclock.
> >
> > When enabled, syscalls that can change time, such as settimeofday(2)
> > or adj_timex(2) are disabled in the guest.
> >
> > Signed-off-by: Suleiman Souhlal <suleiman@google.com>
> > ---
> >  arch/x86/Kconfig                |   9 +++
> >  arch/x86/include/asm/kvmclock.h |   2 +
> >  arch/x86/kernel/kvmclock.c      | 127 +++++++++++++++++++++++++++++++-
> >  kernel/time/timekeeping.c       |  21 ++++++
> >  4 files changed, 155 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> > index 4195f44c6a09..37299377d9d7 100644
> > --- a/arch/x86/Kconfig
> > +++ b/arch/x86/Kconfig
> > @@ -837,6 +837,15 @@ config PARAVIRT_TIME_ACCOUNTING
> >  config PARAVIRT_CLOCK
> >       bool
> >
> > +config KVMCLOCK_HOST_TIMEKEEPING
> > +     bool "kvmclock uses host timekeeping"
> > +     depends on KVM_GUEST
> > +     ---help---
> > +       Select this option to make the guest use the same timekeeping
> > +       parameters as the host. This means that time will be almost
> > +       exactly the same between the two. Only works if the host uses "tsc"
> > +       clocksource.
> > +
>
> I'd also like to speak up against this config, it is confusing. In case
> the goal is to come up with a TSC-based clock for guests which will
> return the same as clock_gettime() on the host (or, is the goal to just
> have the same reading for all guests on the host?) I'd suggest we create
> a separate (from KVMCLOCK) clocksource (mirroring host timekeeper) and
> guests will be free to pick the one they like.

Fair enough. I'll do that in the next version of the patch.

The goal is to have a guest clock that gives the same
clock_gettime(CLOCK_MONOTONIC) as the host.

Thanks,
-- Suleiman