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

[Rorie Reyes <rreyes@linux.ibm.com>]

On 1/7/25 2:06 PM, Alex Williamson wrote:
> On Tue,  7 Jan 2025 13:43:49 -0500
> Rorie Reyes <rreyes@linux.ibm.com> wrote:
>
>> This patch series creates and registers a handler that is called when
>> userspace is notified by the kernel that a guest's AP configuration has
>> changed. The handler in turn notifies the guest that its AP configuration
>> has changed. This allows the guest to immediately respond to AP
>> configuration changes rather than relying on polling or some other
>> inefficient mechanism for detecting config changes.
> 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
>
Hey Alex,

Sorry for the long delay, but to answer your question, the 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 perofrm the I/O.

Our VFIO device does not follow that model and it represents a guest's 
AP configuration, not an individual AP device. Granting

guest access to AP devices in the configuration is controlled outside of 
the VFIO. The purpose of the mdev is to provide

a means for specifying the AP configuration for the guest. When the mdev 
is attached to the guest, the vfio_ap device driver

set the AP configuration assigned to the mdev into the control blocks 
used to start the guest.