Re: [PATCH v3 2/3] vduse: add VDUSE_GET_FEATURES ioctl

[Jason Wang <jasowang@redhat.com>]

On Wed, Mar 11, 2026 at 3:08 AM Eugenio Pérez <eperezma@redhat.com> wrote:
>
> Add an ioctl to allow VDUSE instances to query the available features
> supported by the kernel module.
>
> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> ---
> A simple u64 bitmap is used for feature flags. While a flexible array
> could support indefinite expansion, 64 bits is sufficient for the
> foreseeable future and simplifies the implementation.
>
> Also, dev_dbg is used for logging rather than dev_err as these can be
> triggered from userspace.
> ---
> v3:
> * Remove check for API_VERSION < 2
> * Add comment about struct vduse_dev_config:vduse_features is only valid
>   if VDUSE_GET_FEATURES success.
>
> v2:
> * return -EINVAL if ioctl called with version < 2, so userland visible
>   reply is kept (Jason).
> ---
>  drivers/vdpa/vdpa_user/vduse_dev.c | 7 +++++++
>  include/uapi/linux/vduse.h         | 7 ++++++-
>  2 files changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/vdpa/vdpa_user/vduse_dev.c b/drivers/vdpa/vdpa_user/vduse_dev.c
> index d1da7c15d98b..17e0358d3a68 100644
> --- a/drivers/vdpa/vdpa_user/vduse_dev.c
> +++ b/drivers/vdpa/vdpa_user/vduse_dev.c
> @@ -52,6 +52,9 @@
>
>  #define IRQ_UNBOUND -1
>
> +/* Supported VDUSE features */
> +static const uint64_t vduse_features;
> +
>  /*
>   * VDUSE instance have not asked the vduse API version, so assume 0.
>   *
> @@ -2207,6 +2210,10 @@ static long vduse_ioctl(struct file *file, unsigned int cmd,
>                 ret = vduse_destroy_dev(name);
>                 break;
>         }
> +       case VDUSE_GET_FEATURES:
> +               ret = put_user(vduse_features, (u64 __user *)argp);
> +               break;
> +
>         default:
>                 ret = -EINVAL;
>                 break;
> diff --git a/include/uapi/linux/vduse.h b/include/uapi/linux/vduse.h
> index 361eea511c21..e9b5f32a452d 100644
> --- a/include/uapi/linux/vduse.h
> +++ b/include/uapi/linux/vduse.h
> @@ -33,6 +33,7 @@
>   * @vq_align: the allocation alignment of virtqueue's metadata
>   * @ngroups: number of vq groups that VDUSE device declares
>   * @nas: number of address spaces that VDUSE device declares
> + * @vduse_features: VDUSE features
>   * @reserved: for future use, needs to be initialized to zero
>   * @config_size: the size of the configuration space
>   * @config: the buffer of the configuration space
> @@ -49,7 +50,8 @@ struct vduse_dev_config {
>         __u32 vq_align;
>         __u32 ngroups; /* if VDUSE_API_VERSION >= 1 */
>         __u32 nas; /* if VDUSE_API_VERSION >= 1 */
> -       __u32 reserved[11];
> +       __u64 vduse_features; /* if VDUSE_GET_FEATURES is not EINVAL */
> +       __u32 reserved[9];

If we go with VDUSE_GET_FEATURES, I'd perfer to go with
VDUSE_SET_FEATURES intead of this.

But we can hear from others.

>         __u32 config_size;
>         __u8 config[];
>  };
> @@ -63,6 +65,9 @@ struct vduse_dev_config {
>   */
>  #define VDUSE_DESTROY_DEV      _IOW(VDUSE_BASE, 0x03, char[VDUSE_NAME_MAX])
>
> +/* Get the VDUSE supported features */
> +#define VDUSE_GET_FEATURES     _IOR(VDUSE_BASE, 0x04, __u64)
> +
>  /* The ioctls for VDUSE device (/dev/vduse/$NAME) */
>
>  /**
> --
> 2.53.0
>

Thanks