vhost: Reintroduce kthread API and add mode selection

Since commit 6e890c5d5021 ("vhost: use vhost_tasks for worker threads"),
the vhost uses vhost_task and operates as a child of the
owner thread. This is required for correct CPU usage accounting,
especially when using containers.

However, this change has caused confusion for some legacy
userspace applications, and we didn't notice until it's too late.

Unfortunately, it's too late to revert - we now have userspace
depending both on old and new behaviour :(

To address the issue, reintroduce kthread mode for vhost workers and
provide a configuration to select between kthread and task worker.

- Add 'fork_owner' parameter to vhost_dev to let users select kthread
  or task mode. Default mode is task mode(VHOST_FORK_OWNER_TASK).

- Reintroduce kthread mode support:
  * Bring back the original vhost_worker() implementation,
    and renamed to vhost_run_work_kthread_list().
  * Add cgroup support for the kthread
  * Introduce struct vhost_worker_ops:
    - Encapsulates create / stop / wake‑up callbacks.
    - vhost_worker_create() selects the proper ops according to
      inherit_owner.

- Userspace configuration interface:
  * New IOCTLs:
      - VHOST_SET_FORK_FROM_OWNER lets userspace select task mode
        (VHOST_FORK_OWNER_TASK) or kthread mode (VHOST_FORK_OWNER_KTHREAD)
      - VHOST_GET_FORK_FROM_OWNER reads the current worker mode
  * Expose module parameter 'fork_from_owner_default' to allow system
    administrators to configure the default mode for vhost workers
  * Kconfig option CONFIG_VHOST_ENABLE_FORK_OWNER_CONTROL controls whether
    these IOCTLs and the parameter are available

- The VHOST_NEW_WORKER functionality requires fork_owner to be set
  to true, with validation added to ensure proper configuration

This partially reverts or improves upon:
  commit 6e890c5d5021 ("vhost: use vhost_tasks for worker threads")
  commit 1cdaafa1b8b4 ("vhost: replace single worker pointer with xarray")

Fixes: 6e890c5d5021 ("vhost: use vhost_tasks for worker threads"),
Signed-off-by: Cindy Lu <lulu@redhat.com>
Message-Id: <20250714071333.59794-2-lulu@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Tested-by: Lei Yang <leiyang@redhat.com>

1 files changed, 29 insertions, 0 deletions

diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
index d4b3e2ae1314..e72f2655459e 100644
--- a/include/uapi/linux/vhost.h
+++ b/include/uapi/linux/vhost.h
@@ -235,4 +235,33 @@
  */
 #define VHOST_VDPA_GET_VRING_SIZE	_IOWR(VHOST_VIRTIO, 0x82,	\
 					      struct vhost_vring_state)
+
+/* fork_owner values for vhost */
+#define VHOST_FORK_OWNER_KTHREAD 0
+#define VHOST_FORK_OWNER_TASK 1
+
+/**
+ * VHOST_SET_FORK_FROM_OWNER - Set the fork_owner flag for the vhost device,
+ * This ioctl must called before VHOST_SET_OWNER.
+ * Only available when CONFIG_VHOST_ENABLE_FORK_OWNER_CONTROL=y
+ *
+ * @param fork_owner: An 8-bit value that determines the vhost thread mode
+ *
+ * When fork_owner is set to VHOST_FORK_OWNER_TASK(default value):
+ *   - Vhost will create vhost worker as tasks forked from the owner,
+ *     inheriting all of the owner's attributes.
+ *
+ * When fork_owner is set to VHOST_FORK_OWNER_KTHREAD:
+ *   - Vhost will create vhost workers as kernel threads.
+ */
+#define VHOST_SET_FORK_FROM_OWNER _IOW(VHOST_VIRTIO, 0x83, __u8)
+
+/**
+ * VHOST_GET_FORK_OWNER - Get the current fork_owner flag for the vhost device.
+ * Only available when CONFIG_VHOST_ENABLE_FORK_OWNER_CONTROL=y
+ *
+ * @return: An 8-bit value indicating the current thread mode.
+ */
+#define VHOST_GET_FORK_FROM_OWNER _IOR(VHOST_VIRTIO, 0x84, __u8)
+
 #endif