Re: [PATCH] KVM: x86: move steal time initialization to vcpu entry time

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On Wed, Oct 14, 2015 at 3:33 PM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote:
>
> As reported at https://bugs.launchpad.net/qemu/+bug/1494350,
> it is possible to have vcpu->arch.st.last_steal initialized
> from a thread other than vcpu thread, say the iothread, via
> KVM_SET_MSRS.
>
> Which can cause an overflow later (when subtracting from vcpu threads
> sched_info.run_delay).
>
> To avoid that, move steal time accumulation to vcpu entry time,
> before copying steal time data to guest.
>
> Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>

Reviewed-by: David Matlack <dmatlack@xxxxxxxxxx>

>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 8f0f6ec..0e0332e 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2030,6 +2030,8 @@ static void accumulate_steal_time(struct kvm_vcpu *vcpu)
>
>  static void record_steal_time(struct kvm_vcpu *vcpu)
>  {
> +       accumulate_steal_time(vcpu);
> +
>         if (!(vcpu->arch.st.msr_val & KVM_MSR_ENABLED))
>                 return;
>
> @@ -2182,12 +2184,6 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>                 if (!(data & KVM_MSR_ENABLED))
>                         break;
>
> -               vcpu->arch.st.last_steal = current->sched_info.run_delay;
> -
> -               preempt_disable();
> -               accumulate_steal_time(vcpu);
> -               preempt_enable();
> -
>                 kvm_make_request(KVM_REQ_STEAL_UPDATE, vcpu);
>
>                 break;
> @@ -2830,7 +2826,6 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>                 vcpu->cpu = cpu;
>         }
>
> -       accumulate_steal_time(vcpu);
>         kvm_make_request(KVM_REQ_STEAL_UPDATE, vcpu);
>  }
>
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