On 11/5/24 3:39 AM, Jason Wang wrote:

On Tue, Nov 5, 2024 at 3:28 PM Cindy Lu [off-list ref] wrote:

quoted

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 <redacted>
---
 drivers/vhost/vhost.c      | 15 ++++++++++++++-
 include/uapi/linux/vhost.h |  2 ++
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index c17dc01febcc..70c793b63905 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c

@@ -2274,8 +2274,9 @@ 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;

        /* If you are not the owner, you can become one */
        if (ioctl == VHOST_SET_OWNER) {

@@ -2332,6 +2333,18 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
                if (ctx)
                        eventfd_ctx_put(ctx);
                break;
+       case VHOST_SET_INHERIT_FROM_OWNER:
+               /*inherit_owner can only be modified before owner is set*/
+               if (vhost_dev_has_owner(d))
+                       break;
+
+               if (copy_from_user(&inherit_owner, argp,
+                                  sizeof(inherit_owner))) {
+                       r = -EFAULT;
+                       break;
+               }
+               d->inherit_owner = inherit_owner;
+               break;

Is there any case that we need to switch from owner back to kthread?
If not I would choose a more simplified API that is just
VHOST_INHERIT_OWNER.

I can't think of any need to be able to switch back and forth for
general use.

However for this patchset, I think in patch 9/9 we set the default as:

inherit_owner_default = true

so the default is to use vhost_tasks.

With that code, we would need VHOST_SET_INHERIT_FROM_OWNER so userspace
can set the kernel to use kthreads.

I'm not sure if in the past emails it was resolved what the default would
be.