Re: [PATCH v1 0/5] Report vfio-ap configuration changes

[Rorie Reyes]

On 1/7/25 2:06 PM, Alex Williamson wrote:

> Why are configuration changes to the device allowed while the device is
> in use?
>
> Would a uevent be considered an inefficient mechanism?  Why?
>
> Thanks,
> Alex

I believe a vfio device is typically used to pass through a single I/O 
device, like a VGPU or PCI device. VFIO allows direct access to the 
memory of the underlying device to perform the I/O. Our vfio device does 
not follow that model. It represents a guest's AP configuration, not 
individual AP devices. Granting guest access to the AP devices in the 
configuration is controlled outside of VFIO. The purpose of the mediated 
device is to provide a means for specifying the AP configuration for the 
guest. When the mediated device is attached to the guest, the vfio_ap 
device driver sets the AP configuration assigned to the mdev into the 
control blocks used to start the guest.

We could use the sysfs attributes of the mediated device to do hot 
plug/unplug of devices for the guest. The vfio_ap host device driver 
would perform the hot plug/unplug via an IBM Z mechanism. It was left up 
to the AP bus driver running in the guest to detect these changes via a 
polling mechanism. My patches allow us to notify the AP bus driver 
immediately - via an eventfd to userspace - of those changes which is 
far more efficient than relying on a polling mechanism.