Re: [PATCH v14 08/10] ptp: arm64: Enable ptp_kvm for arm64
From: Marc Zyngier
Date: Sat Sep 05 2020 - 07:34:07 EST
On Fri, 04 Sep 2020 10:27:42 +0100,
Jianyong Wu <jianyong.wu@xxxxxxx> wrote:
>
> Currently, there is no mechanism to keep time sync between guest and host
> in arm64 virtualization environment. 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 in some scenarios,like in cloud envirenment, we ask
> for higher time precision.
>
> kvm ptp clock, which choose the host clock source as a reference
> clock to sync time between guest and host, has been adopted by x86
> which makes the time sync order from milliseconds to nanoseconds.
>
> This patch enables kvm ptp clock for arm64 and improve clock sync precison
> significantly.
>
> Test result comparisons between with kvm ptp clock and without it in arm64
> are as follows. This test derived from the result of command 'chronyc
> sources'. we should take more care 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 see that the clock difference between host and guest
> is in order of ns.
>
> Signed-off-by: Jianyong Wu <jianyong.wu@xxxxxxx>
> ---
> drivers/clocksource/arm_arch_timer.c | 24 +++++++++++++
> drivers/ptp/Kconfig | 2 +-
> drivers/ptp/ptp_kvm_arm64.c | 53 ++++++++++++++++++++++++++++
> 3 files changed, 78 insertions(+), 1 deletion(-)
> create mode 100644 drivers/ptp/ptp_kvm_arm64.c
And I missed that one earlier:
> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
> index d55acffb0b90..aaf286e90092 100644
> --- a/drivers/clocksource/arm_arch_timer.c
> +++ b/drivers/clocksource/arm_arch_timer.c
> @@ -1650,3 +1650,27 @@ 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>
No conditional includes, please.
> +int kvm_arch_ptp_get_crosststamp(unsigned long *cycle, struct timespec64 *ts,
> + struct clocksource **cs)
> +{
> + struct arm_smccc_res hvc_res;
> + ktime_t ktime;
> +
> + /* Currently, linux guest will always use the virtual counter */
> + arm_smccc_1_1_invoke(ARM_SMCCC_VENDOR_HYP_KVM_PTP_FUNC_ID,
> + ARM_PTP_VIRT_COUNTER, &hvc_res);
You don't need to assume anything. This driver already provides you
with the information you need to tell the hypervisor which counter to
use:
if (arch_timer_uses_ppi == ARCH_TIMER_VIRT_PPI)
ptp_counter = ARM_PTP_VIRT_COUNTER;
else
ptp_counter = ARM_PTP_PHYS_COUNTER;
arm_smccc_1_1_invoke(ARM_SMCCC_VENDOR_HYP_KVM_PTP_FUNC_ID,
ptp_counter, &hvc_res);
At least, this is vaguely future proof.
The hypervisor will still have to discriminate between a call between
a call from vEL1 or vEL2 to decide whether to subtract the offset from
the counter value, but that's out of scope for now.
M.
--
Without deviation from the norm, progress is not possible.