Re: [PATCH v2 7/7] vhost: Add new UAPI to support change to task mode

[Stefano Garzarella]

On Fri, Oct 04, 2024 at 09:58:21AM GMT, Cindy Lu wrote:
> Add a new UAPI to support setting the vhost device to
> use task mode. The user space application needs to use
> VHOST_SET_INHERIT_FROM_OWNER to set the mode.
> This setting must be set before VHOST_SET_OWNER is set.

Why?

> Signed-off-by: Cindy Lu <lulu@xxxxxxxxxx>
> ---
> drivers/vhost/vhost.c      | 18 +++++++++++++++++-
> include/uapi/linux/vhost.h |  2 ++
> 2 files changed, 19 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 08c9e77916ca..0e5c81026acd 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -2341,8 +2341,24 @@ 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;
> +	bool inherit_owner;
             ^
             This can be moved ...
> +
> +	if (ioctl == VHOST_SET_INHERIT_FROM_OWNER) {

                ^
                ... here.

> +		/* Is there an owner already? */

This comment is superfluous, I would instead explain why there has to be 
an owner, what problem would we have if not?

> +		if (vhost_dev_has_owner(d)) {
> +			r = -EBUSY;
> +			goto done;
> +		}
> +		if (copy_from_user(&inherit_owner, argp,
> +				   sizeof(inherit_owner))) {
> +			r = -EFAULT;
> +			goto done;
> +		}
> +		enforce_inherit_owner = inherit_owner;

mmmm, I'm confused.

IIUC with this change, an user, for example, can call 
VHOST_SET_INHERIT_FROM_OWNER on /dev/vhost-vsock and change the 
behaviour of /dev/vhost-net.

Is that really what we want?

Maybe it's better if the module parameter controls the default of any 
device, but the ioctl changes the behavior only for that device.

> +		goto done;
> +	}
>
> 	/* If you are not the owner, you can become one */
> 	if (ioctl == VHOST_SET_OWNER) {
> diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
> index b95dd84eef2d..1e192038633d 100644
> --- a/include/uapi/linux/vhost.h
> +++ b/include/uapi/linux/vhost.h
> @@ -235,4 +235,6 @@
>  */
> #define VHOST_VDPA_GET_VRING_SIZE	_IOWR(VHOST_VIRTIO, 0x82,	\
> 					      struct vhost_vring_state)
> +

Please, add documentation here.

> +#define VHOST_SET_INHERIT_FROM_OWNER _IOW(VHOST_VIRTIO, 0x83, bool)
                                       ^
                                       Please, add a tab like we did
                                       for all previous definitions.

Thanks,
Stefano