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

Marc Zyngier
2020-01-07T09:29:37Z

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_FUNC_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?

> + *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_FUNC_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_FUNC_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.

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