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Date:   Wed, 25 Jan 2017 11:03:45 -0200
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Radim Krcmar <rkrcmar@...hat.com>,
        Richard Cochran <richardcochran@...il.com>,
        Miroslav Lichvar <mlichvar@...hat.com>
Subject: Re: [patch 4/4] PTP: add kvm PTP driver

On Wed, Jan 25, 2017 at 01:56:45PM +0100, Paolo Bonzini wrote:
> 
> 
> On 24/01/2017 18:09, Marcelo Tosatti wrote:
> > 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@...hat.com>
> > 
> > ---
> >  drivers/ptp/Kconfig   |   12 ++
> >  drivers/ptp/Makefile  |    1 
> >  drivers/ptp/ptp_kvm.c |  202 ++++++++++++++++++++++++++++++++++++++++++++++++++
> >  3 files changed, 215 insertions(+)
> > 
> > v2: check for kvmclock (Radim)
> >     initialize global variables before device registration (Radim)
> > v3: use cross timestamps callback (Paolo, Miroslav, Radim)
> > v4: remove gettime64 callback (Paolo)
> > v5: drop useless nsec assignment (Radim)
> > 
> > Index: kvm-ptpdriver/drivers/ptp/Kconfig
> > ===================================================================
> > --- kvm-ptpdriver.orig/drivers/ptp/Kconfig	2017-01-24 09:03:47.604586098 -0200
> > +++ kvm-ptpdriver/drivers/ptp/Kconfig	2017-01-24 09:05:06.321704124 -0200
> > @@ -90,4 +90,16 @@
> >  	  To compile this driver as a module, choose M here: the module
> >  	  will be called ptp_pch.
> >  
> > +config PTP_1588_CLOCK_KVM
> > +	tristate "KVM virtual PTP clock"
> > +	depends on PTP_1588_CLOCK
> > +	depends on KVM_GUEST
> > +	default y
> > +	help
> > +	  This driver adds support for using kvm infrastructure as a PTP
> > +	  clock. This clock is only useful if you are using KVM guests.
> > +
> > +	  To compile this driver as a module, choose M here: the module
> > +	  will be called ptp_kvm.
> > +
> >  endmenu
> > Index: kvm-ptpdriver/drivers/ptp/Makefile
> > ===================================================================
> > --- kvm-ptpdriver.orig/drivers/ptp/Makefile	2017-01-24 09:03:47.604586098 -0200
> > +++ kvm-ptpdriver/drivers/ptp/Makefile	2017-01-24 09:05:06.322704125 -0200
> > @@ -6,3 +6,4 @@
> >  obj-$(CONFIG_PTP_1588_CLOCK)		+= ptp.o
> >  obj-$(CONFIG_PTP_1588_CLOCK_IXP46X)	+= ptp_ixp46x.o
> >  obj-$(CONFIG_PTP_1588_CLOCK_PCH)	+= ptp_pch.o
> > +obj-$(CONFIG_PTP_1588_CLOCK_KVM)	+= ptp_kvm.o
> > 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	2017-01-24 09:05:06.322704125 -0200
> > @@ -0,0 +1,202 @@
> > +/*
> > + * 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;
> > +
> > +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);
> 
> Any reason to take the spinlock in the non-preemptable region?
> This would be worse for RT kernels than taking it outside.

Not really, can move it before.

> 
> > +	do {
> > +		/*
> > +		 * We are using a TSC value read in the hosts
> > +		 * kvm_hc_clock_pairing handling.
> > +		 * So any changes to tsc_to_system_mul
> > +		 * and tsc_shift or any other pvclock
> > +		 * data invalidate that 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);
> 
> Need ratelimiting in case something goes wrong and the host stops using
> the TSC clocksource?

Yes wise.


> > +			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;
> 
> Flags written and not read.
> 
> Also, I think I disagree with Radim, gettime64 should use the TSC
> correction.  It's the right thing to do for the POSIX clock interface.
> It's a bit arbitrary for PTP_SYS_OFFSET (though you cannot really argue
> with better precision), but chrony is now using PTP_SYS_OFFSET_PRECISE
> (and again you cannot really argue with better precision anyway).
> 
> I'll test this a bit and then push it to kvm/queue.  Please take a
> look!
> 
> Paolo

Ok cool thanks Paolo, will review and ping if necessary.

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