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73 lines
3.6 KiB
73 lines
3.6 KiB
.. SPDX-License-Identifier: GPL-2.0 |
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Clocks and Timers |
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================= |
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arm64 |
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----- |
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On arm64, Hyper-V virtualizes the ARMv8 architectural system counter |
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and timer. Guest VMs use this virtualized hardware as the Linux |
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clocksource and clockevents via the standard arm_arch_timer.c |
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driver, just as they would on bare metal. Linux vDSO support for the |
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architectural system counter is functional in guest VMs on Hyper-V. |
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While Hyper-V also provides a synthetic system clock and four synthetic |
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per-CPU timers as described in the TLFS, they are not used by the |
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Linux kernel in a Hyper-V guest on arm64. However, older versions |
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of Hyper-V for arm64 only partially virtualize the ARMv8 |
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architectural timer, such that the timer does not generate |
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interrupts in the VM. Because of this limitation, running current |
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Linux kernel versions on these older Hyper-V versions requires an |
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out-of-tree patch to use the Hyper-V synthetic clocks/timers instead. |
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x86/x64 |
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------- |
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On x86/x64, Hyper-V provides guest VMs with a synthetic system clock |
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and four synthetic per-CPU timers as described in the TLFS. Hyper-V |
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also provides access to the virtualized TSC via the RDTSC and |
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related instructions. These TSC instructions do not trap to |
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the hypervisor and so provide excellent performance in a VM. |
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Hyper-V performs TSC calibration, and provides the TSC frequency |
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to the guest VM via a synthetic MSR. Hyper-V initialization code |
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in Linux reads this MSR to get the frequency, so it skips TSC |
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calibration and sets tsc_reliable. Hyper-V provides virtualized |
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versions of the PIT (in Hyper-V Generation 1 VMs only), local |
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APIC timer, and RTC. Hyper-V does not provide a virtualized HPET in |
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guest VMs. |
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The Hyper-V synthetic system clock can be read via a synthetic MSR, |
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but this access traps to the hypervisor. As a faster alternative, |
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the guest can configure a memory page to be shared between the guest |
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and the hypervisor. Hyper-V populates this memory page with a |
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64-bit scale value and offset value. To read the synthetic clock |
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value, the guest reads the TSC and then applies the scale and offset |
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as described in the Hyper-V TLFS. The resulting value advances |
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at a constant 10 MHz frequency. In the case of a live migration |
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to a host with a different TSC frequency, Hyper-V adjusts the |
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scale and offset values in the shared page so that the 10 MHz |
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frequency is maintained. |
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Starting with Windows Server 2022 Hyper-V, Hyper-V uses hardware |
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support for TSC frequency scaling to enable live migration of VMs |
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across Hyper-V hosts where the TSC frequency may be different. |
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When a Linux guest detects that this Hyper-V functionality is |
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available, it prefers to use Linux's standard TSC-based clocksource. |
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Otherwise, it uses the clocksource for the Hyper-V synthetic system |
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clock implemented via the shared page (identified as |
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"hyperv_clocksource_tsc_page"). |
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The Hyper-V synthetic system clock is available to user space via |
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vDSO, and gettimeofday() and related system calls can execute |
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entirely in user space. The vDSO is implemented by mapping the |
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shared page with scale and offset values into user space. User |
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space code performs the same algorithm of reading the TSC and |
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appying the scale and offset to get the constant 10 MHz clock. |
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Linux clockevents are based on Hyper-V synthetic timer 0. While |
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Hyper-V offers 4 synthetic timers for each CPU, Linux only uses |
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timer 0. Interrupts from stimer0 are recorded on the "HVS" line in |
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/proc/interrupts. Clockevents based on the virtualized PIT and |
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local APIC timer also work, but the Hyper-V synthetic timer is |
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preferred. |
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The driver for the Hyper-V synthetic system clock and timers is |
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drivers/clocksource/hyperv_timer.c.
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