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

From: Marc Zyngier <[email-masked]>
Date: Thu, 09 Jan 2020 09:24:20 +0000

On 2020-01-09 05:59, Jianyong Wu wrote:
> Hi Marc,
> 
>> -----Original Message-----
>> From: Marc Zyngier <[email-masked]>
>> Sent: Tuesday, January 7, 2020 5:29 PM
>> To: Jianyong Wu <[email-masked]>
>> Cc: [email-masked]; [email-masked]; [email-masked];
>> [email-masked]; [email-masked]; 
>> [email-masked];
>> [email-masked]; Mark Rutland <[email-masked]>;
>> [email-masked]; Suzuki Poulose <[email-masked]>; Steven Price
>> <[email-masked]>; [email-masked]; linux-arm-
>> [email-masked]; [email-masked];
>> [email-masked]; Steve Capper <[email-masked]>; Kaly Xin
>> <[email-masked]>; Justin He <[email-masked]>; nd <[email-masked]>
>> 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 <[email-masked]>
>> > ---
>> >  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...