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

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Date:   Fri, 13 Jan 2017 10:41:10 -0500
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Radim Krcmar <rkrcmar@...hat.com>, xen-devel@...ts.xenproject.org,
        Joao Martins <joao.m.martins@...cle.com>
Cc:     Marcelo Tosatti <mtosatti@...hat.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 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?

Thanks.
> 
> > +{
> > +	struct pvclock_gtod_data *gtod = &pvclock_gtod_data;
> > +	unsigned long seq;
> > +	int mode;
> > +	u64 ns;
> > +
> > +	do {
> > +		seq = read_seqcount_begin(&gtod->seq);
> > +		mode = gtod->clock.vclock_mode;
> > +		ts->tv_sec = gtod->wall_time_sec;
> > +		ns = gtod->wall_time_snsec;
> > +		ns += vgettsc(cycle_now);
> > +		ns >>= gtod->clock.shift;
> > +	} while (unlikely(read_seqcount_retry(&gtod->seq, seq)));
> > +
> > +	ts->tv_sec += __iter_div_u64_rem(ns, NSEC_PER_SEC, &ns);
> > +	ts->tv_nsec = ns;
> > +
> > +	return mode;
> > +}
> > +
> >  /* returns true if host is using tsc clocksource */
> >  static bool kvm_get_time_and_clockread(s64 *kernel_ns, cycle_t *cycle_now)
> >  {
> > @@ -1632,6 +1659,17 @@
> >  
> >  	return do_monotonic_boot(kernel_ns, cycle_now) == VCLOCK_TSC;
> >  }
> > +
> > +/* returns true if host is using tsc clocksource */
> > +static bool kvm_get_walltime_and_clockread(struct timespec *ts,
> > +					   cycle_t *cycle_now)
> > +{
> > +	/* checked again under seqlock below */
> > +	if (pvclock_gtod_data.clock.vclock_mode != VCLOCK_TSC)
> > +		return false;
> > +
> > +	return do_realtime(ts, cycle_now) == VCLOCK_TSC;
> > +}
> >  #endif
> >  
> >  /*
> > 
> > 
> --
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