[PATCH 04/10] vfio: Use a struct of function pointers instead of a many symbol_get()'s

[Jason Gunthorpe <jgg@xxxxxxxxxx>]

kvm and VFIO need to be coupled together however neither is willing to
tolerate a direct module dependency. Instead when kvm is given a VFIO FD
it uses many symbol_get()'s to access VFIO.

Provide a single VFIO function vfio_file_get_ops() which validates the
given struct file * is a VFIO file and then returns a struct of ops.

Following patches will redo each of the symbol_get() calls into an
indirection through this ops struct.

Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
---
 drivers/vfio/vfio.c  | 19 +++++++++++++++++++
 include/linux/vfio.h |  3 +++
 virt/kvm/vfio.c      | 14 ++++++++++++++
 3 files changed, 36 insertions(+)

diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
index a4555014bd1e72..93508f6a8beda5 100644
--- a/drivers/vfio/vfio.c
+++ b/drivers/vfio/vfio.c
@@ -2013,6 +2013,25 @@ long vfio_external_check_extension(struct vfio_group *group, unsigned long arg)
 }
 EXPORT_SYMBOL_GPL(vfio_external_check_extension);
 
+static const struct vfio_file_ops vfio_file_group_ops = {
+};
+
+/**
+ * vfio_file_get_ops - Return a struct of function pointers to use with the file
+ * @filep: VFIO file to return pointers for
+ *
+ * This API exists to allow KVM to call into VFIO without tightly coupling the
+ * VFIO and KVM modules together. KVM will call this using symbol_get() and from
+ * then on will call VFIO through the returned function pointers.
+ */
+const struct vfio_file_ops *vfio_file_get_ops(struct file *filep)
+{
+	if (filep->f_op != &vfio_group_fops)
+		return ERR_PTR(-EINVAL);
+	return &vfio_file_group_ops;
+}
+EXPORT_SYMBOL_GPL(vfio_file_get_ops);
+
 /*
  * Sub-module support
  */
diff --git a/include/linux/vfio.h b/include/linux/vfio.h
index 66dda06ec42d1b..409bbf817206cc 100644
--- a/include/linux/vfio.h
+++ b/include/linux/vfio.h
@@ -138,6 +138,8 @@ int vfio_mig_get_next_state(struct vfio_device *device,
 /*
  * External user API
  */
+struct vfio_file_ops {
+};
 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 vfio_group *vfio_group_get_external_user_from_dev(struct device
@@ -147,6 +149,7 @@ extern bool vfio_external_group_match_file(struct vfio_group *group,
 extern int vfio_external_user_iommu_id(struct vfio_group *group);
 extern long vfio_external_check_extension(struct vfio_group *group,
 					  unsigned long arg);
+const struct vfio_file_ops *vfio_file_get_ops(struct file *filep);
 
 #define VFIO_PIN_PAGES_MAX_ENTRIES	(PAGE_SIZE/sizeof(unsigned long))
 
diff --git a/virt/kvm/vfio.c b/virt/kvm/vfio.c
index 7e1793a1f5f1fd..254d8c18378163 100644
--- a/virt/kvm/vfio.c
+++ b/virt/kvm/vfio.c
@@ -34,6 +34,7 @@ static void kvm_spapr_tce_release_iommu_group(struct kvm *kvm,
 struct kvm_vfio_group {
 	struct list_head node;
 	struct file *filp;
+	const struct vfio_file_ops *ops;
 	struct vfio_group *vfio_group;
 };
 
@@ -196,6 +197,7 @@ static void kvm_vfio_update_coherency(struct kvm_device *dev)
 
 static int kvm_vfio_group_add(struct kvm_device *dev, unsigned int fd)
 {
+	const struct vfio_file_ops *(*fn)(struct file *filep);
 	struct kvm_vfio *kv = dev->private;
 	struct vfio_group *vfio_group;
 	struct kvm_vfio_group *kvg;
@@ -221,6 +223,18 @@ static int kvm_vfio_group_add(struct kvm_device *dev, unsigned int fd)
 		goto err_unlock;
 	}
 
+	fn = symbol_get(vfio_file_get_ops);
+	if (!fn) {
+		ret = -EINVAL;
+		goto err_free;
+	}
+	kvg->ops = fn(filp);
+	symbol_put(vfio_file_get_ops);
+	if (IS_ERR(kvg->ops)) {
+		ret = PTR_ERR(kvg->ops);
+		goto err_free;
+	}
+
 	vfio_group = kvm_vfio_group_get_external_user(filp);
 	if (IS_ERR(vfio_group)) {
 		ret = PTR_ERR(vfio_group);
-- 
2.35.1