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

[Keith Busch <kbusch@kernel.org>]

On Tue, Jan 28, 2025 at 04:41:41PM +0100, Paolo Bonzini wrote:
> I'm queuing the patch with the store before vhost_task_start, and
> acquire/release instead of just READ_ONCE/WRITE_ONCE.

Thanks, looks good to me:

Reviewed-by: Keith Busch <kbusch@kernel.org>
 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 74c20dbb92da..6d5708146384 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -7127,7 +7127,8 @@ static void kvm_wake_nx_recovery_thread(struct kvm *kvm)
>  	 * 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);
> +	struct vhost_task *nx_thread =
> +		smp_load_acquire(&kvm->arch.nx_huge_page_recovery_thread);
>  	if (nx_thread)
>  		vhost_task_wake(nx_thread);
> @@ -7474,10 +7475,10 @@ static void kvm_mmu_start_lpage_recovery(struct once *once)
>  	if (!nx_thread)
>  		return;
> -	vhost_task_start(nx_thread);
> +	/* Make the task visible only once it is fully created. */
> +	smp_store_release(&kvm->arch.nx_huge_page_recovery_thread, nx_thread);
> -	/* Make the task visible only once it is fully started. */
> -	WRITE_ONCE(kvm->arch.nx_huge_page_recovery_thread, nx_thread);
> +	vhost_task_start(nx_thread);
>  }
>  int kvm_mmu_post_init_vm(struct kvm *kvm)
>