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

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

On Wed, Feb 26, 2025 at 09:46:06AM +0100, Stefano Garzarella wrote:
> Hi Cindy,
> 
> On Wed, 26 Feb 2025 at 07:14, Cindy Lu <lulu@redhat.com> wrote:
> >
> > On Tue, Feb 25, 2025 at 7:31 PM Stefano Garzarella <sgarzare@redhat.com> wrote:
> > >
> > > 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.
> > > >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@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)
> > >
> > > I don't think we really care of the size of the parameter, so can we
> > > just use `bool` or `unsigned int` or `int` for this IOCTL?
> > >
> > > As we did for other IOCTLs where we had to enable/disable something (e.g
> > > VHOST_VSOCK_SET_RUNNING, VHOST_VDPA_SET_VRING_ENABLE).
> > >
> > hi Stefano
> > I initially used it as a boolean, but during the code review, the
> > maintainers considered it was unsuitable for the bool use as the
> 
> I see, indeed I found only 1 case of bool:
> 
> include/uapi/misc/xilinx_sdfec.h:#define XSDFEC_SET_BYPASS
> _IOW(XSDFEC_MAGIC, 9, bool)
> 
> > interface in ioctl (I think in version 3 ?). So I changed it to u8,
> > then will check if this is 1/0 in ioctl and the u8 should be
> > sufficient for us to use
> 
> Okay, if Michael and Jason are happy with it, it's fine.
> It just seemed strange to me that for other IOCTLs we use int or
> unsigned int when we need a boolean instead of a sized type.

I only found VHOST_VSOCK_SET_RUNNING. which other ioctls?

> Thanks for looking at it,
> Stefano