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Date:   Mon, 7 Sep 2020 08:51:23 +0000
From:   Jianyong Wu <Jianyong.Wu@....com>
To:     Marc Zyngier <maz@...nel.org>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "yangbo.lu@....com" <yangbo.lu@....com>,
        "john.stultz@...aro.org" <john.stultz@...aro.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "sean.j.christopherson@...el.com" <sean.j.christopherson@...el.com>,
        "richardcochran@...il.com" <richardcochran@...il.com>,
        Mark Rutland <Mark.Rutland@....com>,
        "will@...nel.org" <will@...nel.org>,
        Suzuki Poulose <Suzuki.Poulose@....com>,
        Steven Price <Steven.Price@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Steve Capper <Steve.Capper@....com>,
        Justin He <Justin.He@....com>, nd <nd@....com>
Subject: RE: [PATCH v14 08/10] ptp: arm64: Enable ptp_kvm for arm64

Hi Marc,

> -----Original Message-----
> From: Marc Zyngier <maz@...nel.org>
> Sent: Saturday, September 5, 2020 7:33 PM
> To: Jianyong Wu <Jianyong.Wu@....com>
> Cc: netdev@...r.kernel.org; yangbo.lu@....com; john.stultz@...aro.org;
> tglx@...utronix.de; pbonzini@...hat.com; sean.j.christopherson@...el.com;
> richardcochran@...il.com; Mark Rutland <Mark.Rutland@....com>;
> will@...nel.org; Suzuki Poulose <Suzuki.Poulose@....com>; Steven Price
> <Steven.Price@....com>; linux-kernel@...r.kernel.org; linux-arm-
> kernel@...ts.infradead.org; kvmarm@...ts.cs.columbia.edu;
> kvm@...r.kernel.org; Steve Capper <Steve.Capper@....com>; Justin He
> <Justin.He@....com>; nd <nd@....com>
> Subject: Re: [PATCH v14 08/10] ptp: arm64: Enable ptp_kvm for arm64
> 
> On Fri, 04 Sep 2020 10:27:42 +0100,
> Jianyong Wu <jianyong.wu@....com> 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@....com>
> > ---
> >  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.
> 
Ok.

> > +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_FU
> NC_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_FU
> NC_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.

Very kind of you!

Thanks
Jianyong 
> 
> 	M.
> 
> --
> Without deviation from the norm, progress is not possible.

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