This patch reintroduces kthread mode for vhost workers and provides
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 (for distros that may
    want to disable them)

- 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")

Signed-off-by: Cindy Lu <redacted>
---
 drivers/vhost/Kconfig      |  17 +++
 drivers/vhost/vhost.c      | 244 ++++++++++++++++++++++++++++++++++---
 drivers/vhost/vhost.h      |  22 ++++
 include/uapi/linux/vhost.h |  29 +++++
 4 files changed, 294 insertions(+), 18 deletions(-)

diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
index 020d4fbb947c..1b3602b1f8e2 100644
--- a/drivers/vhost/Kconfig
+++ b/drivers/vhost/Kconfig

@@ -95,4 +95,21 @@ config VHOST_CROSS_ENDIAN_LEGACY
 
 	  If unsure, say "N".
 
+config VHOST_ENABLE_FORK_OWNER_CONTROL
+	bool "Enable VHOST_ENABLE_FORK_OWNER_CONTROL"
+	default n