Re: [PATCH RFC] vfio/mdev: delay uevent after initialization complete

[Alex Williamson <alex.williamson@redhat.com>]

On Fri,  9 Feb 2018 11:27:16 +0100
Cornelia Huck <cohuck@redhat.com> wrote:

> The registration code first registers the mdev device, and then
> proceeds to populate sysfs. An userspace application that listens
> for the ADD uevent is therefore likely to look for sysfs entries
> that have not yet been created.
> 
> The canonical way to fix this is to use attribute groups that are
> registered by the driver core before it sends the ADD uevent; I
> unfortunately did not find a way to make this work in this case,
> though.
> 
> An alternative approach is to suppress uevents before we register
> with the core and generate the ADD uevent ourselves after the
> sysfs infrastructure is in place.
> 
> Signed-off-by: Cornelia Huck <cohuck@redhat.com>
> ---
> 
> This feels like a band-aid, but I can't figure out how to handle creating
> attribute groups when there's a callback in the parent involved.
> 
> This should address the issue with libvirt's processing of mdevs raised in
> https://www.redhat.com/archives/libvir-list/2018-February/msg00023.html
> - although libvirt will still need to deal with older kernels, of course.
> 
> Best to consider this an untested patch :)

I agree, this feels like a band-aide.  If every device in the kernel
needs to suppress udev events until until some key component is added,
that suggests that either udev is broken in general or not being used
as intended.  Zongyong submitted a different proposal to fix this
here[1].  That proposal seems a bit more sound and has precedence
elsewhere in the kernel.  What do you think of that approach?  We
don't need both afaict.  Thanks,

Alex

[1]https://patchwork.kernel.org/patch/10196197/

> ---
>  drivers/vfio/mdev/mdev_core.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/drivers/vfio/mdev/mdev_core.c b/drivers/vfio/mdev/mdev_core.c
> index 126991046eb7..942e880d8e7f 100644
> --- a/drivers/vfio/mdev/mdev_core.c
> +++ b/drivers/vfio/mdev/mdev_core.c
> @@ -335,6 +335,8 @@ int mdev_device_create(struct kobject *kobj, struct device *dev, uuid_le uuid)
>  	mdev->dev.release = mdev_device_release;
>  	dev_set_name(&mdev->dev, "%pUl", uuid.b);
>  
> +	/* don't notify userspace until we're ready */
> +	dev_set_uevent_suppress(&mdev->dev, 1);
>  	ret = device_register(&mdev->dev);
>  	if (ret) {
>  		put_device(&mdev->dev);
> @@ -350,6 +352,9 @@ int mdev_device_create(struct kobject *kobj, struct device *dev, uuid_le uuid)
>  		mdev_device_remove_ops(mdev, true);
>  		goto create_failed;
>  	}
> +	/* all done, notify userspace */
> +	dev_set_uevent_suppress(&mdev->dev, 0);
> +	kobject_uevent(&mdev->dev.kobj, KOBJ_ADD);
>  
>  	mdev->type_kobj = kobj;
>  	dev_dbg(&mdev->dev, "MDEV: created\n");
> @@ -363,6 +368,10 @@ int mdev_device_create(struct kobject *kobj, struct device *dev, uuid_le uuid)
>  	return ret;
>  
>  create_failed:
> +	/*
> +	 * If we reach this, uevents are still suppressed for mdev->dev,
> +	 * so we don't get a KOBJ_DEL uevent without a previous KOBJ_ADD.
> +	 */
>  	device_unregister(&mdev->dev);
>  
>  create_err: