Re: [PATCH v2] x86/kvm: introduce pv idle time

[He Rongguang]

在 2026/7/21 11:00, He Rongguang 写道:
> Hi, this patch introduces a PV mechanism for guests to publish their
> vCPU idle state and accumulated idle time to the host. This allows
> the host to efficiently determine whether a vCPU is currently in its
> idle loop, and knows vCPU idled for how long. QEMU patch and ARM64
> support patch will follow in a subsequent series.
> 
> The guest writes a GPA pointing to a struct kvm_idle_time via
> MSR_KVM_PV_IDLE_TIME. When entering idle, the guest sets the flag field
> to KVM_PV_VCPU_IDLE. On idle exit, it clears the flag back to
> KVM_PV_VCPU_RUNNING and adds the elapsed idle duration to idle_accum
> (in nanoseconds).
> 
> The host can read the flag at any time through
> kvm_arch_is_vcpu_pv_idle() to make better scheduling or resource
> allocation decisions. For example, host may overcommit those vCPUs
> which are mostly idle. An in-guest agent may be absent, or may not
> report the status in time.
> 
> The accumulated idle time provides visibility into per-vCPU usage for
> monitoring purposes. An in-guest agent can report VM CPU usage, but
> this PV mechanism allows the host to obtain guest CPU usage even when
> no agent is installed. This is especially helpful when the hypervisor
> enables exitless-hlt/mwait (for better guest performance) or when the
> guest uses idle halt-polling, both of which make QEMU vCPU thread usage
> deviate from the actual in-guest vCPU usage.
> 
> This feature is advertised via KVM_FEATURE_PV_IDLE_TIME in CPUID leaf
> 0x40000001. Guests enable it by writing the appropriate MSR during
> initialization, similar to the existing steal time mechanism.

Ping, any feedback on this patch would be appreciated.