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

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Date:   Fri, 13 Jan 2017 13:46:43 -0200
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:     Radim Krcmar <rkrcmar@...hat.com>, xen-devel@...ts.xenproject.org,
        Joao Martins <joao.m.martins@...cle.com>, 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 10:41:10AM -0500, Konrad Rzeszutek Wilk wrote:
> 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.
> > 
> > >  };
> > >  
> > >  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?
> > 
> > If it's ok, then vdata->nsec_base == vdata->wall_time_snsec, so we don't
> > need it.
> > 
> > > +
> > >  	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.)
> 
> 
> PV Clock is hypervisor agnostic so both KVM and Xen can use it. Is this clock
> interface suppose to follow that?

If Xen implements the KVM_HC_CLOCK_OFFSET hypercall, Xen guests can use the 
kvm ptp driver, yes.