[PATCH v4 2/4] vduse: add VDUSE_GET_FEATURES ioctl

["Eugenio Pérez" <eperezma@redhat.com>]

Add an ioctl to allow VDUSE instances to query the available features
supported by the kernel module.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
---
v4:
* Store the features in VDUSE control instead of vduse device config.

v3:
* Remove check for API_VERSION < 2
* Add comment about struct vduse_dev_config:vduse_features is only valid
  if VDUSE_GET_FEATURES success.

v2:
* return -EINVAL if ioctl called with version < 2, so userland visible
  reply is kept (Jason).
---
 drivers/vdpa/vdpa_user/vduse_dev.c | 7 +++++++
 include/uapi/linux/vduse.h         | 3 +++
 2 files changed, 10 insertions(+)

diff --git a/drivers/vdpa/vdpa_user/vduse_dev.c b/drivers/vdpa/vdpa_user/vduse_dev.c
index 2af5cb5e1ec6..e1930e38df7a 100644
--- a/drivers/vdpa/vdpa_user/vduse_dev.c
+++ b/drivers/vdpa/vdpa_user/vduse_dev.c
@@ -52,6 +52,9 @@
 
 #define IRQ_UNBOUND -1
 
+/* Supported VDUSE features */
+static const uint64_t vduse_features;
+
 /*
  * VDUSE instance have not asked the vduse API version, so assume 0.
  *
@@ -2343,6 +2346,10 @@ static long vduse_ioctl(struct file *file, unsigned int cmd,
 		ret = vduse_destroy_dev(name);
 		break;
 	}
+	case VDUSE_GET_FEATURES:
+		ret = put_user(vduse_features, (u64 __user *)argp);
+		break;
+
 	default:
 		ret = -EINVAL;
 		break;
diff --git a/include/uapi/linux/vduse.h b/include/uapi/linux/vduse.h
index 361eea511c21..89aa3b448c0a 100644
--- a/include/uapi/linux/vduse.h
+++ b/include/uapi/linux/vduse.h
@@ -63,6 +63,9 @@ struct vduse_dev_config {
  */
 #define VDUSE_DESTROY_DEV	_IOW(VDUSE_BASE, 0x03, char[VDUSE_NAME_MAX])
 
+/* Get the VDUSE supported features */
+#define VDUSE_GET_FEATURES	_IOR(VDUSE_BASE, 0x04, __u64)
+
 /* The ioctls for VDUSE device (/dev/vduse/$NAME) */
 
 /**
-- 
2.55.0