[PATCH v2 2/7] KVM: arm64: Turn vcpu->arch.pause into a counter

[Fuad Tabba]

On Tue, 29 Sept 2026 at 10:36, Marc Zyngier <maz at kernel.org> wrote:
>
> We have situations where we can call kvm_arm_halt_guest() without
> holding config_lock, which means that halt and resume can overlap in
> funny ways when called from separate contexts, and result in situations
> where a vcpu is resumed while other parts of KVM assume it is halted.
> Consequences are left to the imagination of the reader.
>
> Fix this sorry situation by turning kvm_vcpu_arch::pause into an
> atomic counter, which makes the races described above harmless.
>
> Fixes: 3b92830ad41b2 ("KVM: arm/arm64: implement kvm_arm_[halt,resume]_guest")
> Signed-off-by: Marc Zyngier <maz at kernel.org>
> Cc: stable at vger.kernel.org

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

Cheers,
/fuad


> ---
>  arch/arm64/include/asm/kvm_host.h |  7 +++----
>  arch/arm64/kvm/arm.c              | 21 ++++++++++++---------
>  2 files changed, 15 insertions(+), 13 deletions(-)
>
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 27fe0cd5b2d7a..66ea2372f9870 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -889,11 +889,10 @@ struct kvm_vcpu_arch {
>         /*
>          * Don't run the guest (internal implementation need).
>          *
> -        * Contrary to the flags above, this is set/cleared outside of
> -        * a vcpu context, and thus cannot be mixed with the flags
> -        * themselves (or the flag accesses need to be made atomic).
> +        * Contrary to the flags above, this is updated outside of
> +        * a vcpu context, and thus cannot be mixed with the flags.
>          */
> -       bool pause;
> +       atomic_t pause;
>
>         /*
>          * We maintain more than a single set of debug registers to support
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 5f5dd8bead4b9..9a4871cd796bc 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -561,6 +561,7 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
>         kvm_arm_pvtime_vcpu_init(&vcpu->arch);
>
>         vcpu->arch.hw_mmu = &vcpu->kvm->arch.mmu;
> +       atomic_set(&vcpu->arch.pause, 0);
>
>         /*
>          * This vCPU may have been created after mpidr_data was initialized.
> @@ -835,6 +836,11 @@ int kvm_arch_vcpu_ioctl_set_mpstate(struct kvm_vcpu *vcpu,
>         return ret;
>  }
>
> +static bool vcpu_can_run(struct kvm_vcpu *vcpu)
> +{
> +       return !kvm_arm_vcpu_stopped(vcpu) && !atomic_read(&vcpu->arch.pause);
> +}
> +
>  /**
>   * kvm_arch_vcpu_runnable - determine if the vcpu can be scheduled
>   * @v:         The VCPU pointer
> @@ -850,8 +856,7 @@ int kvm_arch_vcpu_runnable(struct kvm_vcpu *v)
>                       (kvm_timer_should_notify_user(v) ||
>                        kvm_pmu_should_notify_user(v)));
>
> -       return ((irq_lines || kvm_vgic_vcpu_pending_irq(v))
> -               && !kvm_arm_vcpu_stopped(v) && !v->arch.pause);
> +       return ((irq_lines || kvm_vgic_vcpu_pending_irq(v)) && vcpu_can_run(v));
>  }
>
>  bool kvm_arch_vcpu_in_kernel(struct kvm_vcpu *vcpu)
> @@ -1014,7 +1019,7 @@ void kvm_arm_halt_guest(struct kvm *kvm)
>         struct kvm_vcpu *vcpu;
>
>         kvm_for_each_vcpu(i, vcpu, kvm)
> -               vcpu->arch.pause = true;
> +               atomic_inc(&vcpu->arch.pause);
>         kvm_make_all_cpus_request(kvm, KVM_REQ_SLEEP);
>  }
>
> @@ -1024,8 +1029,8 @@ void kvm_arm_resume_guest(struct kvm *kvm)
>         struct kvm_vcpu *vcpu;
>
>         kvm_for_each_vcpu(i, vcpu, kvm) {
> -               vcpu->arch.pause = false;
> -               __kvm_vcpu_wake_up(vcpu);
> +               if (atomic_dec_and_test(&vcpu->arch.pause))
> +                       __kvm_vcpu_wake_up(vcpu);
>         }
>  }
>
> @@ -1033,11 +1038,9 @@ static void kvm_vcpu_sleep(struct kvm_vcpu *vcpu)
>  {
>         struct rcuwait *wait = kvm_arch_vcpu_get_wait(vcpu);
>
> -       rcuwait_wait_event(wait,
> -                          (!kvm_arm_vcpu_stopped(vcpu)) && (!vcpu->arch.pause),
> -                          TASK_INTERRUPTIBLE);
> +       rcuwait_wait_event(wait, vcpu_can_run(vcpu), TASK_INTERRUPTIBLE);
>
> -       if (kvm_arm_vcpu_stopped(vcpu) || vcpu->arch.pause) {
> +       if (!vcpu_can_run(vcpu)) {
>                 /* Awaken to handle a signal, request we sleep again later. */
>                 kvm_make_request(KVM_REQ_SLEEP, vcpu);
>         }
> --
> 2.47.3
>