linux-kernel - Re: [PATCH v7 6/8] vhost: uapi to control task mode (owner vs kthread)

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Message-ID: <4rcrc4prhmca5xnmgmyumxj6oh7buewyx5a2iap7rztvuy32z6@c6v63ysjxctx>
Date: Mon, 3 Mar 2025 09:58:26 +0100
From: Stefano Garzarella <sgarzare@...hat.com>
To: Cindy Lu <lulu@...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 v7 6/8] vhost: uapi to control task mode (owner vs
 kthread)

On Sun, Mar 02, 2025 at 10:32:08PM +0800, Cindy Lu wrote:
>Add a new UAPI to configure the vhost device to use the kthread mode
>The userspace application can use IOCTL VHOST_FORK_FROM_OWNER
>to choose between owner and kthread 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 | 15 +++++++++++++++
> 2 files changed, 35 insertions(+), 2 deletions(-)
>
>diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>index be97028a8baf..ff930c2e5b78 100644
>--- a/drivers/vhost/vhost.c
>+++ b/drivers/vhost/vhost.c
>@@ -1134,7 +1134,7 @@ 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.
>@@ -2287,7 +2287,25 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
> 		r = vhost_dev_set_owner(d);
> 		goto done;
> 	}
>-
>+	if (ioctl == VHOST_FORK_FROM_OWNER) {
>+		u8 inherit_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;
>+		}
>+		if (inherit_owner > 1) {
>+			r = -EINVAL;
>+			goto done;
>+		}
>+		d->inherit_owner = (bool)inherit_owner;
>+		r = 0;
>+		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..547b4fa4c3bd 100644
>--- a/include/uapi/linux/vhost.h
>+++ b/include/uapi/linux/vhost.h
>@@ -235,4 +235,19 @@
>  */
> #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

Should we mention that this IOCTL must be called before VHOST_SET_OWNER?

>+ *
>+ * @param inherit_owner: An 8-bit value that determines the vhost thread mode
>+ *
>+ * When inherit_owner is set to 1(default value):
>+ *   - Vhost will create tasks similar to processes forked from the owner,
>+ *     inheriting all of the owner's attributes..
                                                   ^
nit: there 2 points here

>+ *
>+ * When inherit_owner is set to 0:
>+ *   - Vhost will create tasks as kernel thread
>+ */
>+#define VHOST_FORK_FROM_OWNER _IOW(VHOST_VIRTIO, 0x83, __u8)
>+
> #endif
>-- 
>2.45.0
>