Re: [PATCH v6 5/6] vhost: Add new UAPI to support change to task mode

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

better subject:

vhost: uapi to control task mode (owner vs kthread)


On Sun, Feb 23, 2025 at 11:36:20PM +0800, Cindy Lu wrote:
> Add a new UAPI to enable setting the vhost device to task mode.

better:

Add a new UAPI to configure the vhost device to use the kthread mode


> The userspace application can use VHOST_SET_INHERIT_FROM_OWNER
> to configure the mode

... to either owner or kthread.


> if necessary.
> This setting must be applied before VHOST_SET_OWNER, as the worker
> will be created in the VHOST_SET_OWNER function
> 
> Signed-off-by: Cindy Lu <lulu@redhat.com>
> ---
>  drivers/vhost/vhost.c      | 24 ++++++++++++++++++++++--
>  include/uapi/linux/vhost.h | 18 ++++++++++++++++++
>  2 files changed, 40 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index d8c0ea118bb1..45d8f5c5bca9 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -1133,7 +1133,7 @@ void vhost_dev_reset_owner(struct vhost_dev *dev, struct vhost_iotlb *umem)
>  	int i;
>  
>  	vhost_dev_cleanup(dev);
> -
> +	dev->inherit_owner = true;
>  	dev->umem = umem;
>  	/* We don't need VQ locks below since vhost_dev_cleanup makes sure
>  	 * VQs aren't running.
> @@ -2278,15 +2278,35 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
>  {
>  	struct eventfd_ctx *ctx;
>  	u64 p;
> -	long r;
> +	long r = 0;
>  	int i, fd;
> +	u8 inherit_owner;
>  
>  	/* If you are not the owner, you can become one */
>  	if (ioctl == VHOST_SET_OWNER) {
>  		r = vhost_dev_set_owner(d);
>  		goto done;
>  	}
> +	if (ioctl == VHOST_FORK_FROM_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;
> +		}
> +		/* Validate the inherit_owner value, ensuring it is either 0 or 1 */
> +		if (inherit_owner > 1) {
> +			r = -EINVAL;
> +			goto done;
> +		}
> +
> +		d->inherit_owner = (bool)inherit_owner;
>  
> +		goto done;
> +	}
>  	/* 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 b95dd84eef2d..8f558b433536 100644
> --- a/include/uapi/linux/vhost.h
> +++ b/include/uapi/linux/vhost.h
> @@ -235,4 +235,22 @@
>   */
>  #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
> + *
> + * @param inherit_owner: An 8-bit value that determines the vhost thread mode
> + *
> + * When inherit_owner is set to 1:
> + *   - The VHOST worker threads inherit its values/checks from
> + *     the thread that owns the VHOST device, The vhost threads will
> + *     be counted in the nproc rlimits.
> + *
> + * When inherit_owner is set to 0:
> + *   - The VHOST worker threads will use the traditional kernel thread (kthread)
> + *     implementation, which may be preferred by older userspace applications that
> + *     do not utilize the newer vhost_task concept.
> + */
> +#define VHOST_FORK_FROM_OWNER _IOW(VHOST_VIRTIO, 0x83, __u8)
> +
>  #endif
> -- 
> 2.45.0