On 9/20/24 11:32, 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>
Please have a better run at checkpatch before your next submission, there are still a few ones - most relevant white-space damage.
Anyway this is net-next material... ## Form letter - net-next-closed The merge window for v6.12 and therefore net-next is closed for new drivers, features, code refactoring and optimizations. We are currently accepting bug fixes only. Please repost when net-next reopens after Sept 30th. RFC patches sent for review only are obviously welcome at any time. See: https://www.kernel.org/doc/html/next/process/maintainer-netdev.html#development-cycle -- pw-bot: defer
