iommu/vt-d: Always reserve a domain ID for identity setup

We will use a global static identity domain. Reserve a static domain ID
for it.
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Link: https://lore.kernel.org/r/20240809055431.36513-4-baolu.lu@linux.intel.comSigned-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/intel/iommu.c

@@ -1440,10 +1440,10 @@ static int iommu_init_domains(struct intel_iommu *iommu)
 	 * entry for first-level or pass-through translation modes should
 	 * be programmed with a domain id different from those used for
 	 * second-level or nested translation. We reserve a domain id for
-	 * this purpose.
+	 * this purpose. This domain id is also used for identity domain
+	 * in legacy mode.
 	 */
-	if (sm_supported(iommu))
-		set_bit(FLPT_DEFAULT_DID, iommu->domain_ids);
+	set_bit(FLPT_DEFAULT_DID, iommu->domain_ids);
 
 	return 0;
 }