[PATCH v2 2/3] iommufd/viommu: Publish a vDEVICE only after vdevice_init() succeeds

[Jason Gunthorpe]

On Sun, Jul 05, 2026 at 10:36:10PM -0700, Nicolin Chen wrote:
> @@ -218,18 +218,28 @@ int iommufd_vdevice_alloc_ioctl(struct iommufd_ucmd *ucmd)
>  	 */
>  	idev->vdev = vdev;
>  
> -	curr = xa_cmpxchg(&viommu->vdevs, virt_id, NULL, vdev, GFP_KERNEL);
> -	if (curr) {
> -		rc = xa_err(curr) ?: -EEXIST;
> +	/*
> +	 * Reserve the slot with a zero entry (reads back as NULL) until the
> +	 * vdevice_init() op accepts the vDEVICE. Only the xa_* helpers hide a
> +	 * reserved entry, so never use a raw xas_* iterator on this xarray.
> +	 */
> +	rc = xa_insert(&viommu->vdevs, virt_id, NULL, GFP_KERNEL);
> +	if (rc) {

Why xa_insert() not xa_reserve() ?

Jason