Re: [patch 5/5] PTP: add kvm PTP driver

[Marcelo Tosatti]

On Fri, Jan 20, 2017 at 03:12:56PM +0100, Radim Krcmar wrote:
> 2017-01-20 10:20-0200, Marcelo Tosatti:
> > Add a driver with gettime method returning hosts realtime clock.
> > This allows Chrony to synchronize host and guest clocks with 
> > high precision (see results below).
> > 
> > chronyc> sources
> > MS Name/IP address         Stratum Poll Reach LastRx Last sample
> > ===============================================================================
> > #* PHC0                          0   3   377     4   +162ns[ -683ns] +/-   11ns
> > 
> > To configure Chronyd to use PHC refclock, add the 
> > following line to its configuration file:
> > 
> > refclock PHC /dev/ptpX poll 3 dpoll -2 offset 0
> > 
> > Where /dev/ptpX is the kvmclock PTP clock.
> > 
> > Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
> > 
> > ---
> >  drivers/ptp/Kconfig   |   12 ++
> >  drivers/ptp/Makefile  |    1 
> >  drivers/ptp/ptp_kvm.c |  213 ++++++++++++++++++++++++++++++++++++++++++++++++++
> >  3 files changed, 226 insertions(+)
> > 
> > v2: check for kvmclock (Radim)
> >     initialize global variables before device registration (Radim)
> > v3: use cross timestamps callback (Paolo, Miroslav, Radim)
> > 
> > Index: kvm-ptpdriver/drivers/ptp/ptp_kvm.c
> > ===================================================================
> > --- /dev/null	1970-01-01 00:00:00.000000000 +0000
> > +++ kvm-ptpdriver/drivers/ptp/ptp_kvm.c	2017-01-20 10:19:20.555311672 -0200
> > @@ -0,0 +1,213 @@
> > +/*
> > + * Virtual PTP 1588 clock for use with KVM guests
> > + *
> > + * Copyright (C) 2017 Red Hat Inc.
> > + *
> > + *  This program is free software; you can redistribute it and/or modify
> > + *  it under the terms of the GNU General Public License as published by
> > + *  the Free Software Foundation; either version 2 of the License, or
> > + *  (at your option) any later version.
> > + *
> > + *  This program is distributed in the hope that it will be useful,
> > + *  but WITHOUT ANY WARRANTY; without even the implied warranty of
> > + *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> > + *  GNU General Public License for more details.
> > + *
> > + */
> > +#include <linux/device.h>
> > +#include <linux/err.h>
> > +#include <linux/init.h>
> > +#include <linux/kernel.h>
> > +#include <linux/module.h>
> > +#include <uapi/linux/kvm_para.h>
> > +#include <asm/kvm_para.h>
> > +#include <asm/pvclock.h>
> > +#include <asm/kvmclock.h>
> > +#include <uapi/asm/kvm_para.h>
> > +
> > +#include <linux/ptp_clock_kernel.h>
> > +
> > +struct kvm_ptp_clock {
> > +	struct ptp_clock *ptp_clock;
> > +	struct ptp_clock_info caps;
> > +};
> > +
> > +DEFINE_SPINLOCK(kvm_ptp_lock);
> > +
> > +static struct pvclock_vsyscall_time_info *hv_clock;
> > +
> > +static struct kvm_clock_offset clock_off;
> > +static phys_addr_t clock_off_gpa;
> > +
> > +/*
> > + * system_counterval.cycles: kvmclock value com TSC do host.
> > + * system_counterval.cs: kvmclock clocksource.
> > + * device_time: host realtime clock.
> > + *
> > + */
> > +static int ptp_kvm_get_time_fn(ktime_t *device_time,
> > +			       struct system_counterval_t *system_counter,
> > +			       void *ctx)
> > +{
> > +	unsigned long ret;
> > +	struct timespec64 tspec;
> > +	unsigned version;
> > +	u8 flags;
> > +	int cpu;
> > +	struct pvclock_vcpu_time_info *src;
> > +
> > +	preempt_disable_notrace();
> > +	cpu = smp_processor_id();
> > +	src = &hv_clock[cpu].pvti;
> > +
> > +	spin_lock(&kvm_ptp_lock);
> 
> What does the lock prevent?

Protects access to "struct kvm_clock_offset clock_off".

> > +
> > +	do {
> > +		/*
> > +		 * We are measuring the delay between
> > +		 * kvm_hypercall and rdtsc using TSC,
> > +		 * and converting that delta to
> > +		 * tsc_to_system_mul and tsc_shift
> > +		 * So any changes to tsc_to_system_mul
> > +		 * and tsc_shift in this region
> > +		 * invalidate the measurement.
> > +		 */
> > +		version = pvclock_read_begin(src);
> > +
> > +		ret = kvm_hypercall2(KVM_HC_CLOCK_PAIRING,
> > +				     clock_off_gpa,
> > +				     KVM_CLOCK_PAIRING_WALLCLOCK);
> > +		if (ret != 0) {
> > +			pr_err("clock offset hypercall ret %lu\n", ret);
> > +			spin_unlock(&kvm_ptp_lock);
> > +			preempt_enable_notrace();
> > +			return -EOPNOTSUPP;
> > +		}
> > +
> > +		tspec.tv_sec = clock_off.sec;
> > +		tspec.tv_nsec = clock_off.nsec;
> > +		ret = __pvclock_read_cycles(src, clock_off.tsc);
> > +		flags = src->flags;
> > +	} while (pvclock_read_retry(src, version));
> > +
> > +	preempt_enable_notrace();
> > +
> > +	system_counter->cycles = ret;
> > +	system_counter->cs = get_kvmclock_cs();
> 
> Can't we use clocksource_tsc and just pass the tsc without kvmclock in
> the middle?

No, it has to be the kvmclock value.

> > +	tspec.tv_nsec = tspec.tv_nsec;

Oops, yes, will resend only this patch if there are no further comments.

> 
> (This looks extraneous.)
> 
> Thanks.