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

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Message-ID: <77uzlntuxfzcj4qxggac23g73hslbkstygunqus33hzwrmotzq@f2t22l52xqdo>
Date: Wed, 26 Feb 2025 10:05:02 +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 v6 5/6] vhost: Add new UAPI to support change to task mode

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@...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;
> 	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.

Should we document that this (inherit_owner = 1) is the default, so if 
the user doesn't call VHOST_FORK_FROM_OWNER, this mode will be 
automatically selected?

Thanks,
Stefano

>+ *
>+ * 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)
>+
> #endif
>-- 
>2.45.0
>