RE: [RFC 08/20] vfio/pci: Add VFIO_DEVICE_BIND_IOMMUFD

["Tian, Kevin" <kevin.tian@intel.com>]

> From: David Gibson <david@gibson.dropbear.id.au>
> Sent: Wednesday, September 29, 2021 2:01 PM
> 
> On Sun, Sep 19, 2021 at 02:38:36PM +0800, Liu Yi L wrote:
> > This patch adds VFIO_DEVICE_BIND_IOMMUFD for userspace to bind the
> vfio
> > device to an iommufd. No VFIO_DEVICE_UNBIND_IOMMUFD interface is
> provided
> > because it's implicitly done when the device fd is closed.
> >
> > In concept a vfio device can be bound to multiple iommufds, each hosting
> > a subset of I/O address spaces attached by this device.
> 
> I really feel like this many<->many mapping between devices is going
> to be super-confusing, and therefore make it really hard to be
> confident we have all the rules right for proper isolation.

Based on new discussion on group ownership part (patch06), I feel this
many<->many relationship will disappear. The context fd (either container
or iommufd) will uniquely mark the ownership on a physical device and
its group. With this design it's impractical to have one device bound
to multiple iommufds. Actually I don't think this is a compelling usage
in reality. The previous rationale was that no need to impose such restriction
if no special reason... and now we have a reason. 😊

Jason, are you OK with this simplification?

> 
> That's why I was suggesting a concept like endpoints, to break this
> into two many<->one relationships.  I'm ok if that isn't visible in
> the user API, but I think this is going to be really hard to keep
> track of if it isn't explicit somewhere in the internals.
> 

I think this endpoint concept is represented by ioas_device_info in
patch14:

+/*
+ * An ioas_device_info object is created per each successful attaching
+ * request. A list of objects are maintained per ioas when the address
+ * space is shared by multiple devices.
+ */
+struct ioas_device_info {
+	struct iommufd_device *idev;
+	struct list_head next;
 };

currently it's 1:1 mapping before this object and iommufd_device, 
because no pasid support yet.

We can rename it to struct ioas_endpoint if it makes you feel better.

Thanks
Kevin