ptp: Add support for the AMZNC10C 'vmclock' device

The vmclock device addresses the problem of live migration with
precision clocks. The tolerances of a hardware counter (e.g. TSC) are
typically around ±50PPM. A guest will use NTP/PTP/PPS to discipline that
counter against an external source of 'real' time, and track the precise
frequency of the counter as it changes with environmental conditions.

When a guest is live migrated, anything it knows about the frequency of
the underlying counter becomes invalid. It may move from a host where
the counter running at -50PPM of its nominal frequency, to a host where
it runs at +50PPM. There will also be a step change in the value of the
counter, as the correctness of its absolute value at migration is
limited by the accuracy of the source and destination host's time
synchronization.

In its simplest form, the device merely advertises a 'disruption_marker'
which indicates that the guest should throw away any NTP synchronization
it thinks it has, and start again.

Because the shared memory region can be exposed all the way to userspace
through the /dev/vmclock0 node, applications can still use time from a
fast vDSO 'system call', and check the disruption marker to be sure that
their timestamp is indeed truthful.

The structure also allows for the precise time, as known by the host, to
be exposed directly to guests so that they don't have to wait for NTP to
resync from scratch. The PTP driver consumes this information if present.
Like the KVM PTP clock, this PTP driver can convert TSC-based cross
timestamps into KVM clock values. Unlike the KVM PTP clock, it does so
only when such is actually helpful.

The values and fields are based on the nascent virtio-rtc specification,
and the intent is that a version (hopefully precisely this version) of
this structure will be included as an optional part of that spec. In the
meantime, this driver supports the simple ACPI form of the device which
is being shipped in certain commercial hypervisors (and submitted for
inclusion in QEMU).

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 13 insertions, 0 deletions

diff --git a/drivers/ptp/Kconfig b/drivers/ptp/Kconfig
index 604541dcb320..e98c9767e0ef 100644
--- a/drivers/ptp/Kconfig
+++ b/drivers/ptp/Kconfig
@@ -131,6 +131,19 @@ config PTP_1588_CLOCK_KVM
 	  To compile this driver as a module, choose M here: the module
 	  will be called ptp_kvm.
 
+config PTP_1588_CLOCK_VMCLOCK
+	tristate "Virtual machine PTP clock"
+	depends on X86_TSC || ARM_ARCH_TIMER
+	depends on PTP_1588_CLOCK && ACPI && ARCH_SUPPORTS_INT128
+	default y
+	help
+	  This driver adds support for using a virtual precision clock
+	  advertised by the hypervisor. This clock is only useful in virtual
+	  machines where such a device is present.
+
+	  To compile this driver as a module, choose M here: the module
+	  will be called ptp_vmclock.
+
 config PTP_1588_CLOCK_IDT82P33
 	tristate "IDT 82P33xxx PTP clock"
 	depends on PTP_1588_CLOCK && I2C