On Thu, 2024-12-19 at 18:24 +0100, David Woodhouse wrote: > On Tue, 2024-12-03 at 17:33 +0000, David Woodhouse wrote: > > From: David Woodhouse <d...@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 <d...@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. > > > Ping?
Post-holiday-season ping... ?
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