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H784OdBsWHxMsiPFrK4sRnh2aCT9zaVFHUJRAMYU= On 5/10/26 11:40 AM, David Woodhouse wrote: > On Sun, 2026-05-10 at 18:09 +0100, David Woodhouse wrote: >> On Fri, 2026-05-08 at 15:40 -0700, Sean Christopherson wrote: >>> On Mon, May 04, 2026, Dongli Zhang wrote: >>>> KVM does not support vCPU hotplug. When a vCPU is removed, its >>>> corresponding data structures are not freed by KVM. Instead, QEMU destroys >>>> only the userspace state and the vCPU thread, while the KVM vCPU fd remains >>>> open and parked in QEMU. >>>> >>>> As a result, vcpu->arch.st.last_steal is not reset. >>>> >>>> If the same vCPU is later re-created by QEMU, last_steal retains its old >>>> value, while current->sched_info.run_delay starts from zero since a new >>>> vCPU thread is created. This causes >>>> current->sched_info.run_delay - vcpu->arch.st.last_steal to produce a >>>> large, bogus value. >>>> >>>> Fix this by resetting vcpu->arch.st.last_steal to >>>> current->sched_info.run_delay when KVM steal time is enabled. >>> >>> This is quite arbitrary.  E.g. if userspace hands the vCPU off to a different >>> task without going through QEMU's hotplug dance, then current->sched_info.run_delay >>> will also change. >>> >>> Shouldn't x86 hook kvm_arch_vcpu_run_pid_change() and reset last_steal in there? >> >> I'd like to be sure that we get this right for live update and live migration. >> >> I think we *do* get it right for the Xen runstate info... > > Since I'm adding selftests to my kvmclock branch today... I now *know* > this to be true :) > > > https://git.infradead.org/?p=users/dwmw2/linux.git;a=commitdiff;h=d667349116 > > Looks like Sean is right about the pid change though. Thank you very much for reminder! Base on my understanding of source code, the delta between vcpu->arch.st.last_steal and current->sched_info.run_delay (new thread) won't account downtime if we handle it properly in kvm_arch_vcpu_run_pid_change(). I may also add that validation to the selftest. Thank you very much! Dongli Zhang