Subject: Re: [PATCH v5 5/6] vhost: Add new UAPI to support change to task mode

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 8 Jan 2025 07:20:01 -0500

On Thu, Jan 02, 2025 at 11:36:58AM +0800, Jason Wang wrote:
> On Mon, Dec 30, 2024 at 8:45 PM Cindy Lu <[email-masked]> 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 <[email-masked]>
> > ---
> >  drivers/vhost/vhost.c      | 22 +++++++++++++++++++++-
> >  include/uapi/linux/vhost.h | 19 +++++++++++++++++++
> >  2 files changed, 40 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index ff17c42e2d1a..47c1329360ac 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -2250,15 +2250,35 @@ 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_SET_INHERIT_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;
> > +               }
> 
> Not a native speaker but I wonder if "VHOST_FORK_FROM_OWNER" is better or not.
> 
> > +               /* 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;
> 
> So this allows userspace to reset the owner and toggle the value. This
> seems to be fine, but I wonder if we need to some cleanup in
> vhost_dev_reset_owner() or not. Let's explain this somewhere (probably
> in the commit log).
> 
> >
> > +               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..f5fcf0b25736 100644
> > --- a/include/uapi/linux/vhost.h
> > +++ b/include/uapi/linux/vhost.h
> > @@ -235,4 +235,23 @@
> >   */
> >  #define VHOST_VDPA_GET_VRING_SIZE      _IOWR(VHOST_VIRTIO, 0x82,       \
> >                                               struct vhost_vring_state)
> > +
> > +/**
> > + * VHOST_SET_INHERIT_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 (default behavior):
> > + *   - The VHOST worker threads inherit their 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_SET_INHERIT_FROM_OWNER _IOW(VHOST_VIRTIO, 0x83, __u8)
> > +
> >  #endif
> > --
> > 2.45.0
> 
> Thanks

At this point, make these changes with patches on top pls.