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 architectural system counter 9 and timer. Guest VMs use this virtualized hardware as the Linux 10 clocksource and clockevents via the standard arm_arch_timer.c 11 driver, just as they would on bare metal. Linux vDSO support for the 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 17 architectural timer, such that the timer does not generate 18 interrupts in the VM. Because of this limitation, running current 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. 21 22 x86/x64 23 ------- 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 26 also provides access to the virtualized TSC via the RDTSC and 27 related instructions. These TSC instructions do not trap to 28 the hypervisor and so provide excellent performance in a VM. 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 35 guest VMs. 36 37 The Hyper-V synthetic system clock can be read via a synthetic MSR, 38 but this access traps to the hypervisor. As a faster alternative, 39 the guest can configure a memory page to be shared between the guest 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 42 value, the guest reads the TSC and then applies the scale and offset 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 46 scale and offset values in the shared page so that the 10 MHz 47 frequency is maintained. 48 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 56 "hyperv_clocksource_tsc_page"). 57 58 The Hyper-V synthetic system clock is available to user space via 59 vDSO, and gettimeofday() and related system calls can execute 60 entirely in user space. The vDSO is implemented by mapping the 61 shared page with scale and offset values into user space. User 62 space code performs the same algorithm of reading the TSC and 63 applying the scale and offset to get the constant 10 MHz clock. 64 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 68 results in a VMBus control message that is demultiplexed by 69 vmbus_isr() as described in the Documentation/virt/hyperv/vmbus.rst 70 documentation. In newer versions of Hyper-V, stimer0 interrupts can 71 be mapped to an architectural interrupt, which is referred to as 72 "Direct Mode". Linux prefers to use Direct Mode when available. Since 73 x86/x64 doesn't support per-CPU interrupts, Direct Mode statically 74 allocates an x86 interrupt vector (HYPERV_STIMER0_VECTOR) across all CPUs 75 and explicitly codes it to call the stimer0 interrupt handler. Hence 76 interrupts from stimer0 are recorded on the "HVS" line in /proc/interrupts 77 rather than being associated with a Linux IRQ. Clockevents based on the 78 virtualized PIT and local APIC timer also work, but Hyper-V stimer0 79 is preferred. 80 81 The driver for the Hyper-V synthetic system clock and timers is 82 drivers/clocksource/hyperv_timer.c.
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.