linux-kernel - Re: [PATCH v2 7/7] vhost: Add new UAPI to support change to task mode

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Message-ID: <trrpnhe3qahveeizjlwfp33thudcw5jcmh3uunllucgur4txub@gm2k3p4sx6in>
Date: Mon, 7 Oct 2024 15:37:40 +0200
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
Subject: Re: [PATCH v2 7/7] vhost: Add new UAPI to support change to task mode

On Fri, Oct 04, 2024 at 09:58:21AM GMT, Cindy Lu wrote:
>Add a new UAPI to support setting the vhost device to
>use task mode. The user space application needs to use
>VHOST_SET_INHERIT_FROM_OWNER to set the mode.
>This setting must be set before VHOST_SET_OWNER is set.

Why?

>
>Signed-off-by: Cindy Lu <lulu@...hat.com>
>---
> drivers/vhost/vhost.c      | 18 +++++++++++++++++-
> include/uapi/linux/vhost.h |  2 ++
> 2 files changed, 19 insertions(+), 1 deletion(-)
>
>diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>index 08c9e77916ca..0e5c81026acd 100644
>--- a/drivers/vhost/vhost.c
>+++ b/drivers/vhost/vhost.c
>@@ -2341,8 +2341,24 @@ 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;
>+	bool inherit_owner;
              ^
              This can be moved ...
>+
>+	if (ioctl == VHOST_SET_INHERIT_FROM_OWNER) {

                 ^
                 ... here.

>+		/* Is there an owner already? */

This comment is superfluous, I would instead explain why there has to be 
an owner, what problem would we have if not?

>+		if (vhost_dev_has_owner(d)) {
>+			r = -EBUSY;
>+			goto done;
>+		}
>+		if (copy_from_user(&inherit_owner, argp,
>+				   sizeof(inherit_owner))) {
>+			r = -EFAULT;
>+			goto done;
>+		}
>+		enforce_inherit_owner = inherit_owner;

mmmm, I'm confused.

IIUC with this change, an user, for example, can call 
VHOST_SET_INHERIT_FROM_OWNER on /dev/vhost-vsock and change the 
behaviour of /dev/vhost-net.

Is that really what we want?

Maybe it's better if the module parameter controls the default of any 
device, but the ioctl changes the behavior only for that device.

>+		goto done;
>+	}
>
> 	/* If you are not the owner, you can become one */
> 	if (ioctl == VHOST_SET_OWNER) {
>diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
>index b95dd84eef2d..1e192038633d 100644
>--- a/include/uapi/linux/vhost.h
>+++ b/include/uapi/linux/vhost.h
>@@ -235,4 +235,6 @@
>  */
> #define VHOST_VDPA_GET_VRING_SIZE	_IOWR(VHOST_VIRTIO, 0x82,	\
> 					      struct vhost_vring_state)
>+

Please, add documentation here.

>+#define VHOST_SET_INHERIT_FROM_OWNER _IOW(VHOST_VIRTIO, 0x83, bool)
                                        ^
                                        Please, add a tab like we did
                                        for all previous definitions.

Thanks,
Stefano