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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> Reviewed-by: Paul Durrant <paul@xen.org> Message-Id: <07fd5e2f529098ad4d7cab1423fe9f4a03a9cc14.camel@infradead.org> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com> |
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| asm-m68k | 2 years ago | |
| asm-s390 | 7 years ago | |
| asm-x86 | 9 months ago | |
| drm | 1 month ago | |
| linux | 6 days ago | |
| misc | 7 months ago | |