Hermit does not sleep with multiple CPUs when MWAIT is available

[mkroening]

To reproduce:

use std::thread;
use std::time::Duration;

#[cfg(target_os = "hermit")]
use hermit as _;

fn main() {
	loop {
		thread::sleep(Duration::from_hours(1));
	}
}

Build:

cargo build --target=x86_64-unknown-hermit -Zbuild-std=std,panic_abort --features hermit/smp,hermit/loader --package hello_world

Run without issue (no MWAIT):

qemu-system-x86_64 \
-cpu Skylake-Client \
-smp 2 \
-m 1G \
-display none \
-serial stdio \
-kernel hermit-loader-x86_64-multiboot \
-initrd target/x86_64-unknown-hermit/debug/hello_world

Run with MWAIT (use -cpu max):

qemu-system-x86_64 \
-cpu max \
-smp 2 \
-m 1G \
-display none \
-serial stdio \
-kernel hermit-loader-x86_64-multiboot \
-initrd target/x86_64-unknown-hermit/debug/hello_world

Now, two CPUs are fully utilized.

Is this expected or a bug?

[jounathaen]

I can reproduce on my x86-64 machine with qemu-system-x86_64, but not with qemu-kvm.

[mkroening]

Does your CPU support MWAIT with KVM? Does Hermit print that it detected the MWAIT CPU feature on your system?

The OpenStack cluster and Oscar don't advertise MWAIT inside a KVM VM.

On Oscar's host OS, lscpu does mention mwaitx, but that seems to be something else:

• raw_cpuid::FeatureInfo::has_monitor_mwait
• raw_cpuid::ExtendedProcessorFeatureIdentifiers::has_monitorx_mwaitx

[jounathaen]

Right. KVM does not use MWAIT on my machine.