Re: [RFC PATCH v9 7/8] ptp: arm64: Enable ptp_kvm for arm64

Marc Zyngier
2020-01-09T09:24:29Z

On 2020-01-09 05:59, Jianyong Wu wrote:
> Hi Marc,
>
>> -----Original Message-----
>> From: Marc Zyngier <maz@kernel.org>
>> Sent: Tuesday, January 7, 2020 5:29 PM
>> To: Jianyong Wu <Jianyong.Wu@arm.com>
>> Cc: netdev@vger.kernel.org; yangbo.lu@nxp.com; john.stultz@linaro.org;
>> tglx@linutronix.de; pbonzini@redhat.com;
>> sean.j.christopherson@intel.com;
>> richardcochran@gmail.com; Mark Rutland <Mark.Rutland@arm.com>;
>> will@kernel.org; Suzuki Poulose <Suzuki.Poulose@arm.com>; Steven Price
>> <Steven.Price@arm.com>; linux-kernel@vger.kernel.org; linux-arm-
>> kernel@lists.infradead.org; kvmarm@lists.cs.columbia.edu;
>> kvm@vger.kernel.org; Steve Capper <Steve.Capper@arm.com>; Kaly Xin
>> <Kaly.Xin@arm.com>; Justin He <Justin.He@arm.com>; nd <nd@arm.com>
>> Subject: Re: [RFC PATCH v9 7/8] ptp: arm64: Enable ptp_kvm for arm64
>>
>> On 2019-12-10 03:40, Jianyong Wu wrote:
>> > Currently in arm64 virtualization environment, there is no mechanism
>> > to keep time sync between guest and host. Time in guest will drift
>> > compared with host after boot up as they may both use third party time
>> > sources to correct their time respectively. The time deviation will be
>> > in order of milliseconds but some scenarios ask for higher time
>> > precision, like in cloud envirenment, we want all the VMs running in
>> > the host aquire the same level accuracy from host clock.
>> >
>> > Use of kvm ptp clock, which choose the host clock source clock as a
>> > reference clock to sync time clock between guest and host has been
>> > adopted by x86 which makes the time sync order from milliseconds to
>> > nanoseconds.
>> >
>> > This patch enable kvm ptp on arm64 and we get the similar clock drift
>> > as found with x86 with kvm ptp.
>> >
>> > Test result comparison between with kvm ptp and without it in arm64
>> > are as follows. This test derived from the result of command 'chronyc
>> > sources'. we should take more cure of the last sample column which
>> > shows the offset between the local clock and the source at the last
>> > measurement.
>> >
>> > no kvm ptp in guest:
>> > MS Name/IP address Stratum Poll Reach LastRx Last sample
>> >
>> ==========================================================
>> ==============
>> > ^* dns1.synet.edu.cn 2 6 377 13 +1040us[+1581us] +/-
>> > 21ms
>> > ^* dns1.synet.edu.cn 2 6 377 21 +1040us[+1581us] +/-
>> > 21ms
>> > ^* dns1.synet.edu.cn 2 6 377 29 +1040us[+1581us] +/-
>> > 21ms
>> > ^* dns1.synet.edu.cn 2 6 377 37 +1040us[+1581us] +/-
>> > 21ms
>> > ^* dns1.synet.edu.cn 2 6 377 45 +1040us[+1581us] +/-
>> > 21ms
>> > ^* dns1.synet.edu.cn 2 6 377 53 +1040us[+1581us] +/-
>> > 21ms
>> > ^* dns1.synet.edu.cn 2 6 377 61 +1040us[+1581us] +/-
>> > 21ms
>> > ^* dns1.synet.edu.cn 2 6 377 4 -130us[ +796us] +/-
>> > 21ms
>> > ^* dns1.synet.edu.cn 2 6 377 12 -130us[ +796us] +/-
>> > 21ms
>> > ^* dns1.synet.edu.cn 2 6 377 20 -130us[ +796us] +/-
>> > 21ms
>> >
>> > in host:
>> > MS Name/IP address Stratum Poll Reach LastRx Last sample
>> >
>> ==========================================================
>> ==============
>> > ^* 120.25.115.20 2 7 377 72 -470us[ -603us] +/-
>> > 18ms
>> > ^* 120.25.115.20 2 7 377 92 -470us[ -603us] +/-
>> > 18ms
>> > ^* 120.25.115.20 2 7 377 112 -470us[ -603us] +/-
>> > 18ms
>> > ^* 120.25.115.20 2 7 377 2 +872ns[-6808ns] +/-
>> > 17ms
>> > ^* 120.25.115.20 2 7 377 22 +872ns[-6808ns] +/-
>> > 17ms
>> > ^* 120.25.115.20 2 7 377 43 +872ns[-6808ns] +/-
>> > 17ms
>> > ^* 120.25.115.20 2 7 377 63 +872ns[-6808ns] +/-
>> > 17ms
>> > ^* 120.25.115.20 2 7 377 83 +872ns[-6808ns] +/-
>> > 17ms
>> > ^* 120.25.115.20 2 7 377 103 +872ns[-6808ns] +/-
>> > 17ms
>> > ^* 120.25.115.20 2 7 377 123 +872ns[-6808ns] +/-
>> > 17ms
>> >
>> > The dns1.synet.edu.cn is the network reference clock for guest and
>> > 120.25.115.20 is the network reference clock for host. we can't get
>> > the clock error between guest and host directly, but a roughly
>> > estimated value will be in order of hundreds of us to ms.
>> >
>> > with kvm ptp in guest:
>> > chrony has been disabled in host to remove the disturb by network
>> > clock.
>> >
>> > MS Name/IP address Stratum Poll Reach LastRx Last sample
>> >
>> ==========================================================
>> ==============
>> > * PHC0 0 3 377 8 -7ns[ +1ns] +/-
>> > 3ns
>> > * PHC0 0 3 377 8 +1ns[ +16ns] +/-
>> > 3ns
>> > * PHC0 0 3 377 6 -4ns[ -0ns] +/-
>> > 6ns
>> > * PHC0 0 3 377 6 -8ns[ -12ns] +/-
>> > 5ns
>> > * PHC0 0 3 377 5 +2ns[ +4ns] +/-
>> > 4ns
>> > * PHC0 0 3 377 13 +2ns[ +4ns] +/-
>> > 4ns
>> > * PHC0 0 3 377 12 -4ns[ -6ns] +/-
>> > 4ns
>> > * PHC0 0 3 377 11 -8ns[ -11ns] +/-
>> > 6ns
>> > * PHC0 0 3 377 10 -14ns[ -20ns] +/-
>> > 4ns
>> > * PHC0 0 3 377 8 +4ns[ +5ns] +/-
>> > 4ns
>> >
>> > The PHC0 is the ptp clock which choose the host clock as its source
>> > clock. So we can be sure to say that the clock error between host and
>> > guest is in order of ns.
>> >
>> > Signed-off-by: Jianyong Wu <jianyong.wu@arm.com>
>> > ---
>> > drivers/clocksource/arm_arch_timer.c | 22 ++++++++++++
>> > drivers/ptp/Kconfig | 2 +-
>> > drivers/ptp/ptp_kvm_arm64.c | 53
>> ++++++++++++++++++++++++++++
>> > 3 files changed, 76 insertions(+), 1 deletion(-) create mode 100644
>> > drivers/ptp/ptp_kvm_arm64.c
>> >
>> > diff --git a/drivers/clocksource/arm_arch_timer.c
>> > b/drivers/clocksource/arm_arch_timer.c
>> > index 277846decd33..72260b66f02e 100644
>> > --- a/drivers/clocksource/arm_arch_timer.c
>> > +++ b/drivers/clocksource/arm_arch_timer.c
>> > @@ -1636,3 +1636,25 @@ static int __init arch_timer_acpi_init(struct
>> > acpi_table_header *table) } TIMER_ACPI_DECLARE(arch_timer,
>> > ACPI_SIG_GTDT, arch_timer_acpi_init); #endif
>> > +
>> > +#if IS_ENABLED(CONFIG_PTP_1588_CLOCK_KVM)
>> > +#include <linux/arm-smccc.h>
>> > +int kvm_arch_ptp_get_crosststamp(unsigned long *cycle, struct
>> > timespec64 *ts,
>> > + struct clocksource **cs)
>> > +{
>> > + struct arm_smccc_res hvc_res;
>> > + ktime_t ktime_overall;
>> > +
>> > +
>> arm_smccc_1_1_invoke(ARM_SMCCC_VENDOR_HYP_KVM_PTP_FU
>> NC_ID, &hvc_res);
>> > + if ((long)(hvc_res.a0) < 0)
>> > + return -EOPNOTSUPP;
>> > +
>> > + ktime_overall = hvc_res.a0 << 32 | hvc_res.a1;
>> > + *ts = ktime_to_timespec64(ktime_overall);
>> > + *cycle = hvc_res.a2 << 32 | hvc_res.a3;
>>
>> So why isn't that just a read of the virtual counter, given that what
>> you do in
>> the hypervisor seems to be "cntpct - cntvoff"?
>>
>> What am I missing here?
>>
> We need get clock time and counter cycle at the same time, so we can't
> just read virtual counter
> at guest and must get it from host.

See my comment in my reply to patch #6: *Must* seems like a very strong
word,
and you don't explain *why* that's better than just computing the total
hypercall
cost. Hint: given the frequency of the counter (in the few MHz range) vs
the
frequency of a CPU (in the multiple GHz range, and with an IPC close
enough to 1),
I doubt that you'll see the counter making much progress across a
hypercall.

>
>> > + *cs = &clocksource_counter;
>> > +
>> > + return 0;
>> > +}
>> > +EXPORT_SYMBOL_GPL(kvm_arch_ptp_get_crosststamp);
>> > +#endif
>> > diff --git a/drivers/ptp/Kconfig b/drivers/ptp/Kconfig index
>> > 9b8fee5178e8..3c31ff8eb05f 100644
>> > --- a/drivers/ptp/Kconfig
>> > +++ b/drivers/ptp/Kconfig
>> > @@ -110,7 +110,7 @@ config PTP_1588_CLOCK_PCH config
>> > PTP_1588_CLOCK_KVM
>> > tristate "KVM virtual PTP clock"
>> > depends on PTP_1588_CLOCK
>> > - depends on KVM_GUEST && X86
>> > + depends on KVM_GUEST && X86 || ARM64 && ARM_ARCH_TIMER
>> > default y
>> > help
>> > This driver adds support for using kvm infrastructure as a PTP
>> > diff --git a/drivers/ptp/ptp_kvm_arm64.c b/drivers/ptp/ptp_kvm_arm64.c
>> > new file mode 100644 index 000000000000..f3f957117865
>> > --- /dev/null
>> > +++ b/drivers/ptp/ptp_kvm_arm64.c
>> > @@ -0,0 +1,53 @@
>> > +// SPDX-License-Identifier: GPL-2.0-only
>> > +/*
>> > + * Virtual PTP 1588 clock for use with KVM guests
>> > + * Copyright (C) 2019 ARM Ltd.
>> > + * All Rights Reserved
>> > + */
>> > +
>> > +#include <linux/kernel.h>
>> > +#include <linux/err.h>
>> > +#include <asm/hypervisor.h>
>> > +#include <linux/module.h>
>> > +#include <linux/psci.h>
>> > +#include <linux/arm-smccc.h>
>> > +#include <linux/timecounter.h>
>> > +#include <linux/sched/clock.h>
>> > +#include <asm/arch_timer.h>
>> > +
>> > +int kvm_arch_ptp_init(void)
>> > +{
>> > + struct arm_smccc_res hvc_res;
>> > +
>> > +
>> arm_smccc_1_1_invoke(ARM_SMCCC_VENDOR_HYP_KVM_PTP_FU
>> NC_ID,
>> > + &hvc_res);
>> > + if ((long)(hvc_res.a0) < 0)
>> > + return -EOPNOTSUPP;
>> > +
>> > + return 0;
>> > +}
>> > +
>> > +int kvm_arch_ptp_get_clock_generic(struct timespec64 *ts,
>> > + struct arm_smccc_res *hvc_res) {
>> > + ktime_t ktime_overall;
>> > +
>> > +
>> arm_smccc_1_1_invoke(ARM_SMCCC_VENDOR_HYP_KVM_PTP_FU
>> NC_ID,
>> > + hvc_res);
>> > + if ((long)(hvc_res->a0) < 0)
>> > + return -EOPNOTSUPP;
>> > +
>> > + ktime_overall = hvc_res->a0 << 32 | hvc_res->a1;
>> > + *ts = ktime_to_timespec64(ktime_overall);
>> > +
>> > + return 0;
>> > +}
>> > +
>> > +int kvm_arch_ptp_get_clock(struct timespec64 *ts) {
>> > + struct arm_smccc_res hvc_res;
>> > +
>> > + kvm_arch_ptp_get_clock_generic(ts, &hvc_res);
>> > +
>> > + return 0;
>> > +}
>>
>> I also wonder why this is all arm64 specific, while everything should
>> also work
>> just fine on 32bit.
>>
> ptp_kvm is a feature for cloud computing to keep time consistency from
> container to container and to host on server,

Cloud computing? Never heard of that. Will probably never catch on.

> So we focus it on arm64. Also I have never tested it on arm32 machine
> ( we lack of arm32 machine)

I'm sure your employer can provide you with such a box. I can probably
even tell you which cupboard they are stored in... ;-)

> Do you think it's necessary to enable ptp_kvm on arm32? If so, I can do
> that.

I can't see why we wouldn't, given that it should be a zero effort task
(none of the code here is arch specific).

Thanks,

M.
--
Jazz is not dead. It just smells funny...