[PATCH v2 0/4] KVM: Reset steal time accounting on vCPU pid change

[Fuad Tabba]

Hi Dongli,

On Fri, 4 Sept 2026 at 18:56, Dongli Zhang <dongli.zhang at oracle.com> wrote:
>
> v1: https://lore.kernel.org/all/20260816053630.527528-1-dongli.zhang@oracle.com
>
> v1->v2:
>   - move the last_steal field in the main vcpu structure (suggested by
>     Marc Zyngier).
>   - Reset last_steal from the caller of kvm_arch_vcpu_run_pid_change().

I built and ran the series on QEMU/arm64: the new
steal_time_change_pid selftest and the existing steal_time selftest
both pass, and non-protected and protected guest boots are clean.

Tested-by: Fuad Tabba <fuad.tabba at linux.dev>

Cheers,
/fuad

>
> 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.
>
> For instance, current->sched_info.run_delay can become smaller than
> vcpu->arch.st.last_steal (see line 3804) if a QEMU vCPU is re-added after
> it has previously been removed.
>
> As a result, st->steal restarts from a very small value, close to
> current->sched_info.run_delay.
>
> 3720 static void record_steal_time(struct kvm_vcpu *vcpu)
> 3721 {
> ... ...
> 3803         unsafe_get_user(steal, &st->steal, out);
> 3804         steal += current->sched_info.run_delay -
> 3805                 vcpu->arch.st.last_steal;
> 3806         vcpu->arch.st.last_steal = current->sched_info.run_delay;
> 3807         unsafe_put_user(steal, &st->steal, out);
>
>
> This patchset resets last_steal when the vCPU PID changes, as suggested by
> Sean.
>
> Although David suggested accounting the run_delay left over from the
> previous vCPU PID, this series does not do that. It would be easy to make
> that work if KVM could simply assume every transition is a vCPU PID change.
> In practice, KVM does not always have enough information about the previous
> vCPU PID, e.g. after live migration, unless a new ioctl is introduced. For
> now, this series simply resets last_steal.
>
> Although David also suggested doing the same for Xen-on-KVM vCPUs, this
> series does not reset last_steal for Xen vCPUs. That change itself would
> not be difficult, but Xen uses a different mechanism to account downtime,
> including runnable time and offline time when a vCPU is not running. It may
> therefore need no additional ioctl, or a smaller ioctl extension, to account
> run_delay left over from the previous PID.
>
> As I have access to only x86 and arm64 KVM hosts, I created and validated
> the selftest on those two architectures only.
>
> Dongli Zhang (4)
>   KVM: Move last_steal to common struct kvm_vcpu
>   KVM: Reset last_steal on vCPU pid change
>   KVM: selftests: Test steal time across vCPU pid changes on x86
>   KVM: selftests: Add arm64 coverage for steal time pid changes
>
>  arch/arm64/include/asm/kvm_host.h               |   1 -
>  arch/arm64/kvm/Kconfig                          |   1 +
>  arch/arm64/kvm/pvtime.c                         |   8 +-
>  arch/loongarch/include/asm/kvm_host.h           |   1 -
>  arch/loongarch/kvm/Kconfig                      |   1 +
>  arch/loongarch/kvm/exit.c                       |   2 +-
>  arch/loongarch/kvm/vcpu.c                       |   6 +-
>  arch/riscv/include/asm/kvm_host.h               |   1 -
>  arch/riscv/kvm/Kconfig                          |   1 +
>  arch/riscv/kvm/vcpu_sbi_sta.c                   |  10 +-
>  arch/x86/include/asm/kvm_host.h                 |   1 -
>  arch/x86/kvm/Kconfig                            |   1 +
>  arch/x86/kvm/x86.c                              |   5 +-
>  include/linux/kvm_host.h                        |   4 +
>  tools/testing/selftests/kvm/Makefile.kvm        |   2 +
>  .../selftests/kvm/steal_time_change_pid.c       | 216 +++++++++++++++++++
>  virt/kvm/Kconfig                                |   3 +
>  virt/kvm/kvm_main.c                             |   4 +
>  18 files changed, 248 insertions(+), 20 deletions(-)
>
> base-commit: 8ab1afb2eb246ab15b301cd255b5943d208a93c1
>
> Thank you very much!
>
> Dongli Zhang
>