linux-kernel - Re: [PATCH v4 7/8] vhost: Add new UAPI to support change to task mode

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Message-ID: <CACLfguW=WciC77tRzfjFNLzrKcxUCBRy_z0V9rpFhEOQhDvhiw@mail.gmail.com>
Date: Mon, 30 Dec 2024 20:38:19 +0800
From: Cindy Lu <lulu@...hat.com>
To: Stefano Garzarella <sgarzare@...hat.com>
Cc: jasowang@...hat.com, mst@...hat.com, michael.christie@...cle.com, 
	linux-kernel@...r.kernel.org, virtualization@...ts.linux-foundation.org, 
	netdev@...r.kernel.org
Subject: Re: [PATCH v4 7/8] vhost: Add new UAPI to support change to task mode

On Wed, Dec 11, 2024 at 1:55 AM Stefano Garzarella <sgarzare@...hat.com> wrote:
>
> On Wed, Dec 11, 2024 at 12:41:46AM +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@...hat.com>
> >---
> > drivers/vhost/vhost.c      | 22 +++++++++++++++++++++-
> > include/uapi/linux/vhost.h | 18 ++++++++++++++++++
> > 2 files changed, 39 insertions(+), 1 deletion(-)
> >
> >diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> >index 3e9cb99da1b5..12c3bf3d1ed4 100644
> >--- a/drivers/vhost/vhost.c
> >+++ b/drivers/vhost/vhost.c
> >@@ -2257,15 +2257,35 @@ 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;
> >+              }
> >+              /* 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..d7564d62b76d 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_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:
> >+ *   - 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.
>
> We should mention that this is the default behaviour, so the user does
> not need to call VHOST_SET_INHERIT_FROM_OWNER if the default is okay.
>
sure will fix this
thanks
cindy
> >+ *
> >+ * 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)
>
> Do we really need a parameter? I mean could we just have an IOCTL to set
> the old behavior, since the new one is enabled by default?
>
> Not a strong opinion on that, but just an idea to reduce confusion in
> the user. Anyway, if we want the parameter, maybe we can use int instead
> of u8, since we don't particularly care about the length.
>
> Thanks,
> Stefano
>
I think we could keep this, just in case the userspace app needs to
switch back from kthread mode.
Thanks
Cindy
> >+
> > #endif
> >--
> >2.45.0
> >
>