1 .. SPDX-License-Identifier: GPL-2.0
2
3 Clocks and Timers
4 =================
5
6 arm64
7 -----
8 On arm64, Hyper-V virtualizes the ARMv8 archit
9 and timer. Guest VMs use this virtualized hard
10 clocksource and clockevents via the standard a
11 driver, just as they would on bare metal. Linu
12 architectural system counter is functional in
13 While Hyper-V also provides a synthetic system
14 per-CPU timers as described in the TLFS, they
15 Linux kernel in a Hyper-V guest on arm64. How
16 of Hyper-V for arm64 only partially virtualize
17 architectural timer, such that the timer does
18 interrupts in the VM. Because of this limitati
19 Linux kernel versions on these older Hyper-V v
20 out-of-tree patch to use the Hyper-V synthetic
21
22 x86/x64
23 -------
24 On x86/x64, Hyper-V provides guest VMs with a
25 and four synthetic per-CPU timers as described
26 also provides access to the virtualized TSC vi
27 related instructions. These TSC instructions d
28 the hypervisor and so provide excellent perfor
29 Hyper-V performs TSC calibration, and provides
30 to the guest VM via a synthetic MSR. Hyper-V
31 in Linux reads this MSR to get the frequency,
32 calibration and sets tsc_reliable. Hyper-V pro
33 versions of the PIT (in Hyper-V Generation 1
34 APIC timer, and RTC. Hyper-V does not provide
35 guest VMs.
36
37 The Hyper-V synthetic system clock can be read
38 but this access traps to the hypervisor. As a
39 the guest can configure a memory page to be sh
40 and the hypervisor. Hyper-V populates this me
41 64-bit scale value and offset value. To read t
42 value, the guest reads the TSC and then applie
43 as described in the Hyper-V TLFS. The resultin
44 at a constant 10 MHz frequency. In the case of
45 to a host with a different TSC frequency, Hype
46 scale and offset values in the shared page so
47 frequency is maintained.
48
49 Starting with Windows Server 2022 Hyper-V, Hyp
50 support for TSC frequency scaling to enable li
51 across Hyper-V hosts where the TSC frequency m
52 When a Linux guest detects that this Hyper-V f
53 available, it prefers to use Linux's standard
54 Otherwise, it uses the clocksource for the Hyp
55 clock implemented via the shared page (identif
56 "hyperv_clocksource_tsc_page").
57
58 The Hyper-V synthetic system clock is availabl
59 vDSO, and gettimeofday() and related system ca
60 entirely in user space. The vDSO is implement
61 shared page with scale and offset values into
62 space code performs the same algorithm of read
63 applying the scale and offset to get the const
64
65 Linux clockevents are based on Hyper-V synthet
66 While Hyper-V offers 4 synthetic timers for ea
67 timer 0. In older versions of Hyper-V, an inte
68 results in a VMBus control message that is dem
69 vmbus_isr() as described in the Documentation/
70 documentation. In newer versions of Hyper-V, s
71 be mapped to an architectural interrupt, which
72 "Direct Mode". Linux prefers to use Direct Mod
73 x86/x64 doesn't support per-CPU interrupts, Di
74 allocates an x86 interrupt vector (HYPERV_STIM
75 and explicitly codes it to call the stimer0 in
76 interrupts from stimer0 are recorded on the "H
77 rather than being associated with a Linux IRQ.
78 virtualized PIT and local APIC timer also work
79 is preferred.
80
81 The driver for the Hyper-V synthetic system cl
82 drivers/clocksource/hyperv_timer.c.
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