vfio: Change vfio_external_check_extension() to vfio_file_enforced_coherent()

Instead of a general extension check change the function into a limited
test if the iommu_domain has enforced coherency, which is the only thing
kvm needs to query.

Make the new op self contained by properly refcounting the container
before touching it.
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/5-v3-f7729924a7ea+25e33-vfio_kvm_no_group_jgg@nvidia.comSigned-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/vfio.c

@@ -1701,11 +1701,35 @@ struct iommu_group *vfio_file_iommu_group(struct file *file)
 }
 EXPORT_SYMBOL_GPL(vfio_file_iommu_group);
 
-long vfio_external_check_extension(struct vfio_group *group, unsigned long arg)
+/**
+ * vfio_file_enforced_coherent - True if the DMA associated with the VFIO file
+ *        is always CPU cache coherent
+ * @file: VFIO group file
+ *
+ * Enforced coherency means that the IOMMU ignores things like the PCIe no-snoop
+ * bit in DMA transactions. A return of false indicates that the user has
+ * rights to access additional instructions such as wbinvd on x86.
+ */
+bool vfio_file_enforced_coherent(struct file *file)
 {
-	return vfio_ioctl_check_extension(group->container, arg);
+	struct vfio_group *group = file->private_data;
+	bool ret;
+
+	if (file->f_op != &vfio_group_fops)
+		return true;
+
+	/*
+	 * Since the coherency state is determined only once a container is
+	 * attached the user must do so before they can prove they have
+	 * permission.
+	 */
+	if (vfio_group_add_container_user(group))
+		return true;
+	ret = vfio_ioctl_check_extension(group->container, VFIO_DMA_CC_IOMMU);
+	vfio_group_try_dissolve_container(group);
+	return ret;
 }
-EXPORT_SYMBOL_GPL(vfio_external_check_extension);
+EXPORT_SYMBOL_GPL(vfio_file_enforced_coherent);
 
 /*
  * Sub-module support

include/linux/vfio.h

@@ -139,8 +139,7 @@ int vfio_mig_get_next_state(struct vfio_device *device,
 extern struct vfio_group *vfio_group_get_external_user(struct file *filep);
 extern void vfio_group_put_external_user(struct vfio_group *group);
 extern struct iommu_group *vfio_file_iommu_group(struct file *file);
-extern long vfio_external_check_extension(struct vfio_group *group,
-					  unsigned long arg);
+extern bool vfio_file_enforced_coherent(struct file *file);
 
 #define VFIO_PIN_PAGES_MAX_ENTRIES	(PAGE_SIZE/sizeof(unsigned long))
 

virt/kvm/vfio.c

@@ -75,20 +75,20 @@ static void kvm_vfio_group_set_kvm(struct vfio_group *group, struct kvm *kvm)
 	symbol_put(vfio_group_set_kvm);
 }
 
-static bool kvm_vfio_group_is_coherent(struct vfio_group *vfio_group)
+static bool kvm_vfio_file_enforced_coherent(struct file *file)
 {
-	long (*fn)(struct vfio_group *, unsigned long);
-	long ret;
+	bool (*fn)(struct file *file);
+	bool ret;
 
-	fn = symbol_get(vfio_external_check_extension);
+	fn = symbol_get(vfio_file_enforced_coherent);
 	if (!fn)
 		return false;
 
-	ret = fn(vfio_group, VFIO_DMA_CC_IOMMU);
+	ret = fn(file);
 
-	symbol_put(vfio_external_check_extension);
+	symbol_put(vfio_file_enforced_coherent);
 
-	return ret > 0;
+	return ret;
 }
 
 #ifdef CONFIG_SPAPR_TCE_IOMMU
@@ -136,7 +136,7 @@ static void kvm_vfio_update_coherency(struct kvm_device *dev)
 	mutex_lock(&kv->lock);
 
 	list_for_each_entry(kvg, &kv->group_list, node) {
-		if (!kvm_vfio_group_is_coherent(kvg->vfio_group)) {
+		if (!kvm_vfio_file_enforced_coherent(kvg->file)) {
 			noncoherent = true;
 			break;
 		}