Re: [PATCH net-next v7] ptp: Add support for the AMZNC10C 'vmclock' device

[Richard Cochran]

On Sun, Oct 06, 2024 at 08:17:58AM +0100, David Woodhouse wrote:
> From: David Woodhouse <dwmw@xxxxxxxxxxxx>
> 
> 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@xxxxxxxxxxxx>

Acked-by: Richard Cochran <richardcochran@xxxxxxxxx>