========================================================================
^* 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.

Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Jianyong Wu <jianyong.wu@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20201209060932.212364-8-jianyong.wu@arm.com
---
 drivers/clocksource/arm_arch_timer.c | 34 ++++++++++++++++++++++++++++
 drivers/ptp/Kconfig                  |  2 +-
 drivers/ptp/Makefile                 |  1 +
 drivers/ptp/ptp_kvm_arm.c            | 28 +++++++++++++++++++++++
 4 files changed, 64 insertions(+), 1 deletion(-)
 create mode 100644 drivers/ptp/ptp_kvm_arm.c

diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
index 8f12e223703f..e0f167e5e792 100644
--- a/drivers/clocksource/arm_arch_timer.c
+++ b/drivers/clocksource/arm_arch_timer.c
@@ -25,6 +25,8 @@
 #include <linux/sched/clock.h>
 #include <linux/sched_clock.h>
 #include <linux/acpi.h>
+#include <linux/arm-smccc.h>
+#include <linux/ptp_kvm.h>
 
 #include <asm/arch_timer.h>
 #include <asm/virt.h>
@@ -1659,3 +1661,35 @@ 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
+
+int kvm_arch_ptp_get_crosststamp(u64 *cycle, struct timespec64 *ts,
+				 struct clocksource **cs)
+{
+	struct arm_smccc_res hvc_res;
+	u32 ptp_counter;
+	ktime_t ktime;
+
+	if (!IS_ENABLED(CONFIG_HAVE_ARM_SMCCC_DISCOVERY))
+		return -EOPNOTSUPP;
+
+	if (arch_timer_uses_ppi == ARCH_TIMER_VIRT_PPI)
+		ptp_counter = KVM_PTP_VIRT_COUNTER;
+	else
+		ptp_counter = KVM_PTP_PHYS_COUNTER;
+
+	arm_smccc_1_1_invoke(ARM_SMCCC_VENDOR_HYP_KVM_PTP_FUNC_ID,
+			     ptp_counter, &hvc_res);
+
+	if ((int)(hvc_res.a0) < 0)
+		return -EOPNOTSUPP;
+
+	ktime = (u64)hvc_res.a0 << 32 | hvc_res.a1;
+	*ts = ktime_to_timespec64(ktime);
+	if (cycle)
+		*cycle = (u64)hvc_res.a2 << 32 | hvc_res.a3;
+	if (cs)
+		*cs = &clocksource_counter;
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(kvm_arch_ptp_get_crosststamp);
diff --git a/drivers/ptp/Kconfig b/drivers/ptp/Kconfig
index f2edef0df40f..8c20e524e9ad 100644
--- a/drivers/ptp/Kconfig
+++ b/drivers/ptp/Kconfig
@@ -108,7 +108,7 @@ 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) || (HAVE_ARM_SMCCC_DISCOVERY && ARM_ARCH_TIMER)
 	default y
 	help
 	  This driver adds support for using kvm infrastructure as a PTP
diff --git a/drivers/ptp/Makefile b/drivers/ptp/Makefile
index d11eeb5811d1..8673d1743faa 100644
--- a/drivers/ptp/Makefile
+++ b/drivers/ptp/Makefile
@@ -5,6 +5,7 @@
 
 ptp-y					:= ptp_clock.o ptp_chardev.o ptp_sysfs.o
 ptp_kvm-$(CONFIG_X86)			:= ptp_kvm_x86.o ptp_kvm_common.o
+ptp_kvm-$(CONFIG_HAVE_ARM_SMCCC)	:= ptp_kvm_arm.o ptp_kvm_common.o
 obj-$(CONFIG_PTP_1588_CLOCK)		+= ptp.o
 obj-$(CONFIG_PTP_1588_CLOCK_DTE)	+= ptp_dte.o
 obj-$(CONFIG_PTP_1588_CLOCK_INES)	+= ptp_ines.o
diff --git a/drivers/ptp/ptp_kvm_arm.c b/drivers/ptp/ptp_kvm_arm.c
new file mode 100644
index 000000000000..b7d28c8dfb84
--- /dev/null
+++ b/drivers/ptp/ptp_kvm_arm.c
@@ -0,0 +1,28 @@
+// 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/arm-smccc.h>
+#include <linux/ptp_kvm.h>
+
+#include <asm/arch_timer.h>
+#include <asm/hypervisor.h>
+
+int kvm_arch_ptp_init(void)
+{
+	int ret;
+
+	ret = kvm_arm_hyp_service_available(ARM_SMCCC_KVM_FUNC_PTP);
+	if (ret <= 0)
+		return -EOPNOTSUPP;
+
+	return 0;
+}
+
+int kvm_arch_ptp_get_clock(struct timespec64 *ts)
+{
+	return kvm_arch_ptp_get_crosststamp(NULL, ts, NULL);
+}
