vfio: Hold a reference to the iommu_group in kvm for SPAPR

SPAPR exists completely outside the normal iommu driver framework, the
groups it creates are fake and are only created to enable VFIO's uAPI.

Thus, it does not need to follow the iommu core rule that the iommu_group
will only be touched while a driver is attached.

Carry a group reference into KVM and have KVM directly manage the lifetime
of this object independently of VFIO. This means KVM no longer relies on
the vfio group file being valid to maintain the group reference.

Tested-by: Matthew Rosato <mjrosato@linux.ibm.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/2-v2-15417f29324e+1c-vfio_group_disassociate_jgg@nvidia.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/vfio_main.c

@@ -1552,8 +1552,9 @@ static const struct file_operations vfio_device_fops = {
  * vfio_file_iommu_group - Return the struct iommu_group for the vfio group file
  * @file: VFIO group file
  *
- * The returned iommu_group is valid as long as a ref is held on the file.
- * This function is deprecated, only the SPAPR path in kvm should call it.
+ * The returned iommu_group is valid as long as a ref is held on the file. This
+ * returns a reference on the group. This function is deprecated, only the SPAPR
+ * path in kvm should call it.
  */
 struct iommu_group *vfio_file_iommu_group(struct file *file)
 {
@@ -1564,6 +1565,7 @@ struct iommu_group *vfio_file_iommu_group(struct file *file)
 
 	if (!vfio_file_is_group(file))
 		return NULL;
+	iommu_group_ref_get(group->iommu_group);
 	return group->iommu_group;
 }
 EXPORT_SYMBOL_GPL(vfio_file_iommu_group);

virt/kvm/vfio.c

@@ -24,6 +24,9 @@
 struct kvm_vfio_group {
 	struct list_head node;
 	struct file *file;
+#ifdef CONFIG_SPAPR_TCE_IOMMU
+	struct iommu_group *iommu_group;
+#endif
 };
 
 struct kvm_vfio {
@@ -97,12 +100,12 @@ static struct iommu_group *kvm_vfio_file_iommu_group(struct file *file)
 static void kvm_spapr_tce_release_vfio_group(struct kvm *kvm,
 					     struct kvm_vfio_group *kvg)
 {
-	struct iommu_group *grp = kvm_vfio_file_iommu_group(kvg->file);
-
-	if (WARN_ON_ONCE(!grp))
+	if (WARN_ON_ONCE(!kvg->iommu_group))
 		return;
 
-	kvm_spapr_tce_release_iommu_group(kvm, grp);
+	kvm_spapr_tce_release_iommu_group(kvm, kvg->iommu_group);
+	iommu_group_put(kvg->iommu_group);
+	kvg->iommu_group = NULL;
 }
 #endif
 
@@ -252,19 +255,19 @@ static int kvm_vfio_group_set_spapr_tce(struct kvm_device *dev,
 	mutex_lock(&kv->lock);
 
 	list_for_each_entry(kvg, &kv->group_list, node) {
-		struct iommu_group *grp;
-
 		if (kvg->file != f.file)
 			continue;
 
-		grp = kvm_vfio_file_iommu_group(kvg->file);
-		if (WARN_ON_ONCE(!grp)) {
-			ret = -EIO;
-			goto err_fdput;
+		if (!kvg->iommu_group) {
+			kvg->iommu_group = kvm_vfio_file_iommu_group(kvg->file);
+			if (WARN_ON_ONCE(!kvg->iommu_group)) {
+				ret = -EIO;
+				goto err_fdput;
+			}
 		}
 
 		ret = kvm_spapr_tce_attach_iommu_group(dev->kvm, param.tablefd,
-						       grp);
+						       kvg->iommu_group);
 		break;
 	}