On Fri, 10 Sep 2021 10:38:50 -0300 Jason Gunthorpe jgg@nvidia.com wrote:

On Fri, Sep 10, 2021 at 01:10:46PM +0100, Christoph Hellwig wrote:

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On Thu, Sep 09, 2021 at 04:38:45PM -0300, Jason Gunthorpe wrote:

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Every driver just emits a static string, simply feed it through the ops and provide a standard sysfs show function.

Looks sensible. But can you make the attribute optional and add a comment marking it deprecated? Because it really is completely useless. We don't version userspace APIs, userspae has to discover new features individually by e.g. finding new sysfs files or just trying new ioctls.

To be honest I have no idea what side effects that would have..

device code search tells me libvirt reads it and stuffs it into some XML

Something called mdevctl touches it, feeds it into some JSON and other stuff..

qemu has some VFIO_DEVICE_API_* constants but it is all dead code

I agree it shouldn't have been there in the first place

Cornelia? Alex? Any thoughts?

It's not a version, it's a means for userspace to determine the basic API for an mdev device without needing to go through the process of creating a container, adding the group, setting an IOMMU type, opening the device before being able to call VFIO_DEVICE_GET_INFO to determine the API. For example, it wouldn't make sense for libvirt to attach a vfio-ccw device to a PCIe root port in a VM. It's a means to say this mdev device is a vfio-pci or that mdev device is a vfio-ccw. If it were optional, then management tools would have no basic idea how to attach the device to a VM without gaining access to the device themselves. Thanks,

Alex