Re: [PATCH v5] hw/acpi: Add vmclock device

[David Woodhouse]

On Tue, 2024-12-03 at 17:33 +0000, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> 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.
> 
> The device exposes a shared memory region to guests, which can be mapped
> all the way to userspace. In the first phase, this 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 region can be exposed all the way to userspace, 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 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, a simple ACPI device along the lines of VMGENID is perfectly
> sufficient and is compatible with what's being shipped in certain
> commercial hypervisors.
> 
> Linux guest support was merged into the 6.13-rc1 kernel:
> https://git.kernel.org/torvalds/c/205032724226
> 
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> ---
> This is basically unchanged since the last time the structure was
> changed in July, apart from fairly trivial cosmetic changes and now
> importing the header file from Linux.


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