[libvirt] [Question] nodedev-list --cap return nothing after creating a mdev

[Erik Skultety]

On Thu, Feb 01, 2018 at 07:19:45AM +0000, Wuzongyong (Euler Dept) wrote:
> Hi,
>
> I meet a problem that libvirt handle new mediated devices wrongly.
> Command `virsh nodedev-list -cap mdev` return none after I create a mdev, and the root cause I found is:
>
> // kernel code
> int mdev_device_create(struct kobject *kobj, struct device *dev, uuid_le uuid)
> {
> ...
>       ret = device_register(&mdev->dev); // send udev event here --- 1
>       if (ret) {
>             put_device(&mdev->dev);
>             goto create_err;
>       }
>
>       ret = mdev_device_create_ops(kobj, mdev);
>       if (ret)
>             goto create_failed;
>
>       ret = mdev_create_sysfs_files(&mdev->dev, type); // create mdev_type symbol link here --- 2
>       if (ret) {
>             mdev_device_remove_ops(mdev, true);
>             goto create_failed;
>       }
>
>       mdev->type_kobj = kobj;
>       dev_dbg(&mdev->dev, "MDEV: created\n");
> ...
> }
>
> At point 1, kernel send a udev event(add). If libvirt receive this event and handle it before kernel code reach point 2, libvirt
> just fail to resolve link such as '/sys/devices/pci0000:80/0000:80:02.0/0000:81:00.0/36b79575-ada8-4406-893d-8cfd8d10a984/mdev_type'
> because the link haven't been created yet. Then libvirt thinks this devices doesn't exsit and stop tracking info of this device.
> Finally, command `vish nodedev-list -cap` return nothing though the device exist at host indeed.
>
> To make it easy to reproduce this problem, I just add a `udelay(1)` before point 2 by kprobe.
>
> So I think maybe we should add 'kobject_uevent(..., KOBJ_ADD)' after point 2 instead of point 1 ?
> Or is there any other method to solve this problem?

Hi,
just a note, we worked around this in upstream libvirt already.

Thanks for the pointer though, I'd also be interested in possible solutions in
kernel.

Erik