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

[Nicolin Chen]

On Mon, Jul 06, 2026 at 02:23:56PM -0300, Jason Gunthorpe wrote:
> 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() ?

xa_reserve's kdoc says:
"* If there is already something stored at @index, this function does
 * nothing"

By its implementation, it wouldn't return an error on duplication?

Nicolin