linux-kernel - Re: [patch 1/3] KVM: x86: provide realtime host clock via vsyscall notifiers

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Date:   Fri, 13 Jan 2017 13:34:08 -0200
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Radim Krcmar <rkrcmar@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Richard Cochran <richardcochran@...il.com>,
        Miroslav Lichvar <mlichvar@...hat.com>
Subject: Re: [patch 1/3] KVM: x86: provide realtime host clock via vsyscall
 notifiers

On Fri, Jan 13, 2017 at 04:18:04PM +0100, Radim Krcmar wrote:
> 2017-01-13 10:01-0200, Marcelo Tosatti:
> > Expose the realtime host clock and save the TSC value
> > used for the clock calculation.
> > 
> > Signed-off-by: Marcelo Tosatti <mtosatti@...hat.com>
> > 
> > ---
> >  arch/x86/kvm/x86.c |   38 ++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 38 insertions(+)
> > 
> > Index: kvm-ptpdriver/arch/x86/kvm/x86.c
> > ===================================================================
> > --- kvm-ptpdriver.orig/arch/x86/kvm/x86.c	2017-01-13 08:59:03.015895353 -0200
> > +++ kvm-ptpdriver/arch/x86/kvm/x86.c	2017-01-13 09:04:46.581415259 -0200
> > @@ -1139,6 +1139,8 @@
> >  
> >  	u64		boot_ns;
> >  	u64		nsec_base;
> > +	u64		wall_time_sec;
> > +	u64		wall_time_snsec;
> 
> The leading "s" in "snsec" looks like a copy-paste residue.

Just copying the userspace vsyscall interface.

> >  };
> >  
> >  static struct pvclock_gtod_data pvclock_gtod_data;
> > @@ -1162,6 +1164,9 @@
> >  	vdata->boot_ns			= boot_ns;
> >  	vdata->nsec_base		= tk->tkr_mono.xtime_nsec;
> >  
> > +	vdata->wall_time_sec            = tk->xtime_sec;
> > +	vdata->wall_time_snsec          = tk->tkr_mono.xtime_nsec;
> 
> Using tk->tkr_mono offsets for real time seems wrong -- what happens if
> the real time is half a second shifted from monotonic time?

Both the userspace vsyscall interface and getnstimeofday
use it for realtime clock.

Monotonic clock adds the offset:

        vdata->monotonic_time_snsec     = tk->tkr_mono.xtime_nsec
                                        +
((u64)tk->wall_to_monotonic.tv_nsec
                                                << tk->tkr_mono.shift);

> If it's ok, then vdata->nsec_base == vdata->wall_time_snsec, so we don't
> need it.

Just copying the userspace vsyscall interface.

Do you actually want to change the "s" and unify wall_time_snsec with
nsec_base?

> > +
> >  	write_seqcount_end(&vdata->seq);
> >  }
> >  #endif
> > @@ -1623,6 +1628,28 @@
> >  	return mode;
> >  }
> >  
> > +static int do_realtime(struct timespec *ts, cycle_t *cycle_now)
> 
> This is too similar to do_monotonic_boot(), but I don't see a solution
> that is both nice and efficient. :(
> 
> (It usually means macros or copying pvclock_gtod_data.)

Yep.