High VM-EXIT and Host CPU usage on idle with Windows Server 2025
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@lucius: The timers for Windows 11 are per core, so more cores is more (apparent) CPU usage. On 2 vCPUs you won't see the issue, I only see the issue appearing beyond 4 cores (hardware or virtual). Also dependent on how old your hardware is and what other settings you have.
useplatformtick yes - forces the kernel to maintain ticking clocks (like HPET or ACPI)
useplatformclock no - disables HPET
TSC is a tickless clock AND you're disabling HPET, so is Windows falling back to ACPI clocks? Not sure what this ultimately 'does', someone has to go in and poke around with some low level Windows utilities (because Microsoft kernel is weird). This *should* increase your power usage, because now your kernel continues waking up to take care of timers, but this may reduce your *apparent* CPU usage as reported in various statistics as your CPU is continuously 'at full power' and thus doesn't have to wake (which can take hundreds of clock cycles) to service a timer.
IF that 'fixes' the problem, you'll likely see issues especially in gaming/timing/audio applications and significant latency increases in things like VDI. I guess if you're worried about pure energy efficiency you should really take care and measure, but I'm assuming for most people this will result in significant performant impacts for workloads where timing/clocks are necessary (anything with networking, audio and interactivity).
useplatformtick yes - forces the kernel to maintain ticking clocks (like HPET or ACPI)
useplatformclock no - disables HPET
TSC is a tickless clock AND you're disabling HPET, so is Windows falling back to ACPI clocks? Not sure what this ultimately 'does', someone has to go in and poke around with some low level Windows utilities (because Microsoft kernel is weird). This *should* increase your power usage, because now your kernel continues waking up to take care of timers, but this may reduce your *apparent* CPU usage as reported in various statistics as your CPU is continuously 'at full power' and thus doesn't have to wake (which can take hundreds of clock cycles) to service a timer.
IF that 'fixes' the problem, you'll likely see issues especially in gaming/timing/audio applications and significant latency increases in things like VDI. I guess if you're worried about pure energy efficiency you should really take care and measure, but I'm assuming for most people this will result in significant performant impacts for workloads where timing/clocks are necessary (anything with networking, audio and interactivity).
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https://lwn.net/Articles/1051782/
I'm starting to wonder if AMD will fix this, too.
This is for Intel, right?
Code:
This series introduces support for Intel Mode-Based Execute Control
(MBEC) to KVM and nested VMX virtualization. By exposing MBEC to L2
guests, it enables a dramatic reduction in VMexits (up to 24x) for
Windows guests running with Hypervisor-Protected Code Integrity (HVCI),
significantly improving virtualization performance.I'm starting to wonder if AMD will fix this, too.
This is for Intel, right?
I can confirm the same. For windows 11 24H2 with all updates installed. I have VBS enabled and also WSL2 installed(but not working!). I am using x86-64-v3 + nested-virt for cpu flags. The kernel version is 6.17.4-2. It is still about 100% increase of idle cpu but not 300/400 %. WSL2 does not like x86-64-X. I am still playing with this to understand what is the best option.
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