Re: [PATCH RESEND v10 3/3] vhost: Add new UAPI to select kthread mode and KConfig to enable this IOCTL

["Michael S. Tsirkin" <mst@redhat.com>]

On Sat, May 31, 2025 at 05:57:28PM +0800, Cindy Lu wrote:
> This patch introduces a new UAPI that allows the vhost device to select
> in kthread mode. Userspace applications can utilize IOCTL
> VHOST_FORK_FROM_OWNER to select between task and kthread modes, which
> must be invoked before IOCTL VHOST_SET_OWNER, as the worker will be
> created during that call.
> 
> The VHOST_NEW_WORKER requires the inherit_owner setting to be true, and
> a check has been added to ensure proper configuration.
> 
> Additionally, a new KConfig option, CONFIG_VHOST_ENABLE_FORK_OWNER_IOCTL,
> is introduced to control the availability of the IOCTL
> VHOST_FORK_FROM_OWNER. When CONFIG_VHOST_ENABLE_FORK_OWNER_IOCTL is set
> to n, the IOCTL is disabled, and any attempt to use it will result in a
> failure.
> 
> Signed-off-by: Cindy Lu <lulu@redhat.com>

I propose renaming
CONFIG_VHOST_ENABLE_FORK_OWNER_IOCTL
to
CONFIG_VHOST_ENABLE_FORK_OWNER_CONTROL
and it should also control the presence of the module parameter
and a get ioctl (more on which below).

Otherwise we can get a situation where task mode is disabled and
there is no way for userspace to override or check.



> ---
>  drivers/vhost/Kconfig      | 13 +++++++++++++
>  drivers/vhost/vhost.c      | 30 +++++++++++++++++++++++++++++-
>  include/uapi/linux/vhost.h | 16 ++++++++++++++++
>  3 files changed, 58 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
> index 020d4fbb947c..300e474b60fd 100644
> --- a/drivers/vhost/Kconfig
> +++ b/drivers/vhost/Kconfig
> @@ -96,3 +96,16 @@ config VHOST_CROSS_ENDIAN_LEGACY
>  	  If unsure, say "N".
>  
>  endif
> +
> +config VHOST_ENABLE_FORK_OWNER_IOCTL
> +	bool "Enable IOCTL VHOST_FORK_FROM_OWNER"
> +	default n
> +	help
> +	  This option enables the IOCTL VHOST_FORK_FROM_OWNER, allowing
> +	  userspace applications to modify the thread mode for vhost devices.
> +
> +	  By default, `CONFIG_VHOST_ENABLE_FORK_OWNER_IOCTL` is set to `n`,
> +	  which disables the IOCTL. When enabled (y), the IOCTL allows
> +	  users to set the mode as needed.
> +
> +	  If unsure, say "N".
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 2d2909be1bb2..cfa60dc438f9 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -1022,6 +1022,13 @@ long vhost_worker_ioctl(struct vhost_dev *dev, unsigned int ioctl,
>  	switch (ioctl) {
>  	/* dev worker ioctls */
>  	case VHOST_NEW_WORKER:
> +		/*
> +		 * vhost_tasks will account for worker threads under the parent's
> +		 * NPROC value but kthreads do not. To avoid userspace overflowing
> +		 * the system with worker threads inherit_owner must be true.
> +		 */
> +		if (!dev->inherit_owner)
> +			return -EFAULT;
>  		ret = vhost_new_worker(dev, &state);
>  		if (!ret && copy_to_user(argp, &state, sizeof(state)))
>  			ret = -EFAULT;
> @@ -1138,7 +1145,7 @@ void vhost_dev_reset_owner(struct vhost_dev *dev, struct vhost_iotlb *umem)
>  	int i;
>  
>  	vhost_dev_cleanup(dev);
> -
> +	dev->inherit_owner = inherit_owner_default;
>  	dev->umem = umem;
>  	/* We don't need VQ locks below since vhost_dev_cleanup makes sure
>  	 * VQs aren't running.
> @@ -2292,6 +2299,27 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
>  		goto done;
>  	}
>  
> +#ifdef CONFIG_VHOST_ENABLE_FORK_OWNER_IOCTL
> +	if (ioctl == VHOST_FORK_FROM_OWNER) {
> +		u8 inherit_owner;
> +		/*inherit_owner can only be modified before owner is set*/
> +		if (vhost_dev_has_owner(d)) {
> +			r = -EBUSY;
> +			goto done;
> +		}
> +		if (copy_from_user(&inherit_owner, argp, sizeof(u8))) {
> +			r = -EFAULT;
> +			goto done;
> +		}
> +		if (inherit_owner > 1) {
> +			r = -EINVAL;
> +			goto done;
> +		}
> +		d->inherit_owner = (bool)inherit_owner;
> +		r = 0;
> +		goto done;
> +	}
> +#endif
>  	/* You must be the owner to do anything else */
>  	r = vhost_dev_check_owner(d);
>  	if (r)
> diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
> index d4b3e2ae1314..d2692c7ef450 100644
> --- a/include/uapi/linux/vhost.h
> +++ b/include/uapi/linux/vhost.h
> @@ -235,4 +235,20 @@
>   */
>  #define VHOST_VDPA_GET_VRING_SIZE	_IOWR(VHOST_VIRTIO, 0x82,	\
>  					      struct vhost_vring_state)
> +
> +/**
> + * VHOST_FORK_FROM_OWNER - Set the inherit_owner flag for the vhost device,
> + * This ioctl must called before VHOST_SET_OWNER.
> + *
> + * @param inherit_owner: An 8-bit value that determines the vhost thread mode
> + *
> + * When inherit_owner is set to 1(default value):
> + *   - Vhost will create tasks similar to processes forked from the owner,
> + *     inheriting all of the owner's attributes.
> + *
> + * When inherit_owner is set to 0:
> + *   - Vhost will create tasks as kernel thread.
> + */
> +#define VHOST_FORK_FROM_OWNER _IOW(VHOST_VIRTIO, 0x83, __u8)



Given default now depends on the module parameter, we should
have both GET and SET ioctls. All controlled by the kconfig knob.

> +
>  #endif
> -- 
> 2.45.0