virt/hyperv/clocks.rst

1 .. SPDX-License-Identifier: GPL-2.0
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8 On arm64, Hyper-V virtualizes the ARMv8 architectural system counter
12 architectural system counter is functional in guest VMs on Hyper-V.
13 While Hyper-V also provides a synthetic system clock and four synthetic
14 per-CPU timers as described in the TLFS, they are not used by the
15 Linux kernel in a Hyper-V guest on arm64.  However, older versions
16 of Hyper-V for arm64 only partially virtualize the ARMv8
19 Linux kernel versions on these older Hyper-V versions requires an
20 out-of-tree patch to use the Hyper-V synthetic clocks/timers instead.
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24 On x86/x64, Hyper-V provides guest VMs with a synthetic system clock
25 and four synthetic per-CPU timers as described in the TLFS. Hyper-V
29 Hyper-V performs TSC calibration, and provides the TSC frequency
30 to the guest VM via a synthetic MSR.  Hyper-V initialization code
31 in Linux reads this MSR to get the frequency, so it skips TSC
32 calibration and sets tsc_reliable. Hyper-V provides virtualized
33 versions of the PIT (in Hyper-V  Generation 1 VMs only), local
34 APIC timer, and RTC. Hyper-V does not provide a virtualized HPET in
37 The Hyper-V synthetic system clock can be read via a synthetic MSR,
40 and the hypervisor.  Hyper-V populates this memory page with a
41 64-bit scale value and offset value. To read the synthetic clock
43 as described in the Hyper-V TLFS. The resulting value advances
44 at a constant 10 MHz frequency. In the case of a live migration
45 to a host with a different TSC frequency, Hyper-V adjusts the
47 frequency is maintained.
49 Starting with Windows Server 2022 Hyper-V, Hyper-V uses hardware
50 support for TSC frequency scaling to enable live migration of VMs
51 across Hyper-V hosts where the TSC frequency may be different.
52 When a Linux guest detects that this Hyper-V functionality is
53 available, it prefers to use Linux's standard TSC-based clocksource.
54 Otherwise, it uses the clocksource for the Hyper-V synthetic system
55 clock implemented via the shared page (identified as
58 The Hyper-V synthetic system clock is available to user space via
59 vDSO, and gettimeofday() and related system calls can execute
63 applying the scale and offset to get the constant 10 MHz clock.
65 Linux clockevents are based on Hyper-V synthetic timer 0 (stimer0).
66 While Hyper-V offers 4 synthetic timers for each CPU, Linux only uses
67 timer 0. In older versions of Hyper-V, an interrupt from stimer0
70 documentation. In newer versions of Hyper-V, stimer0 interrupts can
73 x86/x64 doesn't support per-CPU interrupts, Direct Mode statically
78 virtualized PIT and local APIC timer also work, but Hyper-V stimer0
81 The driver for the Hyper-V synthetic system clock and timers is