[PATCH v6 10/27] vfio: Add API to open cdev device for Live Update restore

[Vipin Sharma <vipinsh@google.com>]

During Live Update restore, VFIO PCI driver needs to recreate the device
file context that was preserved before kexec and return it to the user.

Provide an API for VFIO Live Update to get VFIO device file. Use an
anonymous inode when creating the file, since the retrieved device file
is not opened through any particular cdev inode, and the cdev inode does
not matter in practice.

Co-developed-by: David Matlack <dmatlack@google.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Signed-off-by: Vipin Sharma <vipinsh@google.com>
---
 drivers/vfio/device_cdev.c | 39 ++++++++++++++++++++++++++++++++++++--
 include/linux/vfio.h       |  6 ++++++
 2 files changed, 43 insertions(+), 2 deletions(-)

diff --git a/drivers/vfio/device_cdev.c b/drivers/vfio/device_cdev.c
index d83e99137e1a..38e691eceab5 100644
--- a/drivers/vfio/device_cdev.c
+++ b/drivers/vfio/device_cdev.c
@@ -2,6 +2,7 @@
 /*
  * Copyright (c) 2023 Intel Corporation.
  */
+#include <linux/anon_inodes.h>
 #include <linux/vfio.h>
 #include <linux/iommufd.h>
 
@@ -20,9 +21,10 @@ void vfio_init_device_cdev(struct vfio_device *device)
 	device->cdev.owner = THIS_MODULE;
 }
 
-static int vfio_device_cdev_open(struct vfio_device *device, struct file *file)
+static int vfio_device_cdev_open(struct vfio_device *device, struct file **filep)
 {
 	struct vfio_device_file *df;
+	struct file *file = *filep;
 	int ret;
 
 	/* Paired with the put in vfio_device_fops_release() */
@@ -40,6 +42,24 @@ static int vfio_device_cdev_open(struct vfio_device *device, struct file *file)
 		goto err_put_registration;
 	}
 
+	/*
+	 * Simulate opening the character device using an anonymous inode. The
+	 * returned file has the same properties as a cdev file (e.g. operations
+	 * are blocked until BIND_IOMMUFD is called).
+	 */
+	if (!file) {
+		file = anon_inode_getfile_fmode("[vfio-device-liveupdate]",
+						&vfio_device_fops, NULL,
+						O_RDWR, FMODE_PREAD | FMODE_PWRITE);
+
+		if (IS_ERR(file)) {
+			ret = PTR_ERR(file);
+			goto err_free_device_file;
+		}
+
+		*filep = file;
+	}
+
 	file->private_data = df;
 
 	/*
@@ -51,11 +71,26 @@ static int vfio_device_cdev_open(struct vfio_device *device, struct file *file)
 
 	return 0;
 
+err_free_device_file:
+	kfree(df);
 err_put_registration:
 	vfio_device_put_registration(device);
 	return ret;
 }
 
+struct file *vfio_device_liveupdate_cdev_open(struct vfio_device *device)
+{
+	struct file *file = NULL;
+	int ret;
+
+	ret = vfio_device_cdev_open(device, &file);
+	if (ret)
+		return ERR_PTR(ret);
+
+	return file;
+}
+EXPORT_SYMBOL_GPL(vfio_device_liveupdate_cdev_open);
+
 /*
  * device access via the fd opened by this function is blocked until
  * .open_device() is called successfully during BIND_IOMMUFD.
@@ -65,7 +100,7 @@ int vfio_device_fops_cdev_open(struct inode *inode, struct file *file)
 	struct vfio_device *device = container_of(inode->i_cdev,
 						  struct vfio_device, cdev);
 
-	return vfio_device_cdev_open(device, file);
+	return vfio_device_cdev_open(device, &file);
 }
 
 static void vfio_df_get_kvm_safe(struct vfio_device_file *df)
diff --git a/include/linux/vfio.h b/include/linux/vfio.h
index 1c1e41de49b3..f46796b6724c 100644
--- a/include/linux/vfio.h
+++ b/include/linux/vfio.h
@@ -434,11 +434,17 @@ void vfio_virqfd_flush_thread(struct virqfd **pvirqfd);
 
 #if IS_ENABLED(CONFIG_VFIO_DEVICE_CDEV)
 bool vfio_file_is_bound_cdev(struct file *file);
+struct file *vfio_device_liveupdate_cdev_open(struct vfio_device *device);
 #else
 static inline bool vfio_file_is_bound_cdev(struct file *file)
 {
 	return false;
 }
+
+static inline struct file *vfio_device_liveupdate_cdev_open(struct vfio_device *device)
+{
+	return ERR_PTR(-EOPNOTSUPP);
+}
 #endif /* IS_ENABLED(CONFIG_VFIO_DEVICE_CDEV) */
 
 struct vfio_device *vfio_find_device(const void *data, device_match_t match);
-- 
2.56.0.317.g454f8302e1-goog