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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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