Re: [PATCH 2/5] KVM: arm64: pvtime: Fix potential loss of stolen time

[Marc Zyngier <maz@kernel.org>]

On 2020-07-11 11:04, Andrew Jones wrote:
> We should only check current->sched_info.run_delay once when
> updating stolen time. Otherwise there's a chance there could
> be a change between checks that we miss (preemption disabling
> comes after vcpu request checks).
> 
> Signed-off-by: Andrew Jones <drjones@redhat.com>
> ---
>  arch/arm64/kvm/pvtime.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/pvtime.c b/arch/arm64/kvm/pvtime.c
> index 2b22214909be..db5ef097a166 100644
> --- a/arch/arm64/kvm/pvtime.c
> +++ b/arch/arm64/kvm/pvtime.c
> @@ -13,6 +13,7 @@
>  void kvm_update_stolen_time(struct kvm_vcpu *vcpu)
>  {
>  	struct kvm *kvm = vcpu->kvm;
> +	u64 last_steal = vcpu->arch.steal.last_steal;
>  	u64 steal;
>  	__le64 steal_le;
>  	u64 offset;
> @@ -24,8 +25,8 @@ void kvm_update_stolen_time(struct kvm_vcpu *vcpu)
> 
>  	/* Let's do the local bookkeeping */
>  	steal = vcpu->arch.steal.steal;
> -	steal += current->sched_info.run_delay - vcpu->arch.steal.last_steal;
>  	vcpu->arch.steal.last_steal = current->sched_info.run_delay;
> +	steal += vcpu->arch.steal.last_steal - last_steal;
>  	vcpu->arch.steal.steal = steal;
> 
>  	steal_le = cpu_to_le64(steal);

Unless you read current->sched_info.run_delay using READ_ONCE,
there is no guarantee that this will do what you want. The
compiler is free to rejig this anyway it wants.

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...