Re: [PATCH] KVM: x86/mmu: Ensure NX huge page recovery thread is alive before waking

[Keith Busch <kbusch@kernel.org>]

On Fri, Jan 24, 2025 at 03:46:23PM -0800, Sean Christopherson wrote:
> When waking a VM's NX huge page recovery thread, ensure the thread is
> actually alive before trying to wake it.  Now that the thread is spawned
> on-demand during KVM_RUN, a VM without a recovery thread is reachable via
> the related module params.

Oh, this is what I thought we could do. I should have read ahead. :)

> +static void kvm_wake_nx_recovery_thread(struct kvm *kvm)
> +{
> +	/*
> +	 * The NX recovery thread is spawned on-demand at the first KVM_RUN and
> +	 * may not be valid even though the VM is globally visible.  Do nothing,
> +	 * as such a VM can't have any possible NX huge pages.
> +	 */
> +	struct vhost_task *nx_thread = READ_ONCE(kvm->arch.nx_huge_page_recovery_thread);
> +
> +	if (nx_thread)
> +		vhost_task_wake(nx_thread);
> +}
> +
>  static int get_nx_huge_pages(char *buffer, const struct kernel_param *kp)
>  {
>  	if (nx_hugepage_mitigation_hard_disabled)
> @@ -7180,7 +7193,7 @@ static int set_nx_huge_pages(const char *val, const struct kernel_param *kp)
>  			kvm_mmu_zap_all_fast(kvm);
>  			mutex_unlock(&kvm->slots_lock);
>  
> -			vhost_task_wake(kvm->arch.nx_huge_page_recovery_thread);
> +			kvm_wake_nx_recovery_thread(kvm);
>  		}
>  		mutex_unlock(&kvm_lock);
>  	}
> @@ -7315,7 +7328,7 @@ static int set_nx_huge_pages_recovery_param(const char *val, const struct kernel
>  		mutex_lock(&kvm_lock);
>  
>  		list_for_each_entry(kvm, &vm_list, vm_list)
> -			vhost_task_wake(kvm->arch.nx_huge_page_recovery_thread);
> +			kvm_wake_nx_recovery_thread(kvm);
>  
>  		mutex_unlock(&kvm_lock);
>  	}
> @@ -7451,14 +7464,20 @@ static void kvm_mmu_start_lpage_recovery(struct once *once)
>  {
>  	struct kvm_arch *ka = container_of(once, struct kvm_arch, nx_once);
>  	struct kvm *kvm = container_of(ka, struct kvm, arch);
> +	struct vhost_task *nx_thread;
>  
>  	kvm->arch.nx_huge_page_last = get_jiffies_64();
> -	kvm->arch.nx_huge_page_recovery_thread = vhost_task_create(
> -		kvm_nx_huge_page_recovery_worker, kvm_nx_huge_page_recovery_worker_kill,
> -		kvm, "kvm-nx-lpage-recovery");
> +	nx_thread = vhost_task_create(kvm_nx_huge_page_recovery_worker,
> +				      kvm_nx_huge_page_recovery_worker_kill,
> +				      kvm, "kvm-nx-lpage-recovery");
>  
> -	if (kvm->arch.nx_huge_page_recovery_thread)
> -		vhost_task_start(kvm->arch.nx_huge_page_recovery_thread);
> +	if (!nx_thread)
> +		return;
> +
> +	vhost_task_start(nx_thread);
> +
> +	/* Make the task visible only once it is fully started. */
> +	WRITE_ONCE(kvm->arch.nx_huge_page_recovery_thread, nx_thread);

I believe the WRITE_ONCE needs to happen before the vhost_task_start to
ensure the parameter update callback can see it before it's started.