Re: [PATCH v2 10/10] vfio/pci: Add VFIO_DEVICE_GET_PCI_HOT_RESET_GROUP_INFO

[Jason Gunthorpe <jgg@nvidia.com>]

On Wed, Mar 29, 2023 at 09:49:44AM -0600, Alex Williamson wrote:
 
> > We could extend bind_iommufd to return the group id or introduce a
> > new ioctl to query it per dev_id.
> 
> That would be ironic to go to all this trouble to remove groups from
> the API only to have them show up here.

Groups always had to be part of the API for advanced cases like qemu -
the point was to make them a small side bit of information not front
and center in control of everything.

> For example, devices within a group cannot be bound to separate
> iommufds due to lack of isolation, which is handled via DMA ownership,
> but barring DMA aliasing issues, due to conventional PCI buses or
> quirks, cdev could allow devices within the same group to be managed by
> separate IOAS's.  

Maybe some future kernel could do this, the API allows it at least..

> So the group information really isn't enough for
> userspace to infer address space restrictions with cdev anyway.
> 
> Therefore aren't we expecting this to be denied at attach_ioas() and
> QEMU shouldn't be making these sorts of assumptions for cdev anyway?

I guess we could make an API specifically to report same-iommu_domina
information?

I was assuming qemu would use the group for now as I don't see a
likely future when we would relax that restriction.. So I was keeping
a "add it when we need it" attitude here.

Jason