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

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Message-ID: <CACGkMEuocDajb0uANEOCLpXsi47Ga+d5K=oF12gDgLRfC2rJSA@mail.gmail.com>
Date: Mon, 24 Feb 2025 09:54:08 +0800
From: Jason Wang <jasowang@...hat.com>
To: Cindy Lu <lulu@...hat.com>
Cc: mst@...hat.com, michael.christie@...cle.com, sgarzare@...hat.com, 
	linux-kernel@...r.kernel.org, virtualization@...ts.linux-foundation.org, 
	netdev@...r.kernel.org
Subject: Re: [PATCH v6 5/6] vhost: Add new UAPI to support change to task mode

On Sun, Feb 23, 2025 at 11:41 PM Cindy Lu <lulu@...hat.com> wrote:
>
> Add a new UAPI to enable setting the vhost device to task mode.
> The userspace application can use VHOST_SET_INHERIT_FROM_OWNER
> to configure the mode 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@...hat.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;

Any reason this needs to be changed under reset_owner?

>         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 */

Code explains itself, let's just drop this comment.

> +               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.

Since this is uAPI, it's better to avoid mentioning too many
implementation details. So I would tweak this as.

"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:
> + *   - 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.

"Vhost will create tasks as kernel thread."

> + */
> +#define VHOST_FORK_FROM_OWNER _IOW(VHOST_VIRTIO, 0x83, __u8)
> +
>  #endif
> --
> 2.45.0
>

Thanks