Clocks and Timers¶
On arm64, Hyper-V virtualizes the ARMv8 architectural system counter and timer. Guest VMs use this virtualized hardware as the Linux clocksource and clockevents via the standard arm_arch_timer.c driver, just as they would on bare metal. Linux vDSO support for the architectural system counter is functional in guest VMs on Hyper-V. While Hyper-V also provides a synthetic system clock and four synthetic per-CPU timers as described in the TLFS, they are not used by the Linux kernel in a Hyper-V guest on arm64. However, older versions of Hyper-V for arm64 only partially virtualize the ARMv8 architectural timer, such that the timer does not generate interrupts in the VM. Because of this limitation, running current Linux kernel versions on these older Hyper-V versions requires an out-of-tree patch to use the Hyper-V synthetic clocks/timers instead.
On x86/x64, Hyper-V provides guest VMs with a synthetic system clock and four synthetic per-CPU timers as described in the TLFS. Hyper-V also provides access to the virtualized TSC via the RDTSC and related instructions. These TSC instructions do not trap to the hypervisor and so provide excellent performance in a VM. Hyper-V performs TSC calibration, and provides the TSC frequency to the guest VM via a synthetic MSR. Hyper-V initialization code in Linux reads this MSR to get the frequency, so it skips TSC calibration and sets tsc_reliable. Hyper-V provides virtualized versions of the PIT (in Hyper-V Generation 1 VMs only), local APIC timer, and RTC. Hyper-V does not provide a virtualized HPET in guest VMs.
The Hyper-V synthetic system clock can be read via a synthetic MSR, but this access traps to the hypervisor. As a faster alternative, the guest can configure a memory page to be shared between the guest and the hypervisor. Hyper-V populates this memory page with a 64-bit scale value and offset value. To read the synthetic clock value, the guest reads the TSC and then applies the scale and offset as described in the Hyper-V TLFS. The resulting value advances at a constant 10 MHz frequency. In the case of a live migration to a host with a different TSC frequency, Hyper-V adjusts the scale and offset values in the shared page so that the 10 MHz frequency is maintained.
Starting with Windows Server 2022 Hyper-V, Hyper-V uses hardware support for TSC frequency scaling to enable live migration of VMs across Hyper-V hosts where the TSC frequency may be different. When a Linux guest detects that this Hyper-V functionality is available, it prefers to use Linux's standard TSC-based clocksource. Otherwise, it uses the clocksource for the Hyper-V synthetic system clock implemented via the shared page (identified as "hyperv_clocksource_tsc_page").
The Hyper-V synthetic system clock is available to user space via vDSO, and gettimeofday() and related system calls can execute entirely in user space. The vDSO is implemented by mapping the shared page with scale and offset values into user space. User space code performs the same algorithm of reading the TSC and applying the scale and offset to get the constant 10 MHz clock.
Linux clockevents are based on Hyper-V synthetic timer 0. While Hyper-V offers 4 synthetic timers for each CPU, Linux only uses timer 0. Interrupts from stimer0 are recorded on the "HVS" line in /proc/interrupts. Clockevents based on the virtualized PIT and local APIC timer also work, but the Hyper-V synthetic timer is preferred.
The driver for the Hyper-V synthetic system clock and timers is drivers/clocksource/hyperv_timer.c.