74 lines
3.6 KiB
ReStructuredText
74 lines
3.6 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
|
|
|
Clocks and Timers
|
|
=================
|
|
|
|
arm64
|
|
-----
|
|
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.
|
|
|
|
x86/x64
|
|
-------
|
|
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.
|