Re: [Qemu-devel] RFC: vfio / device assignment -- layout of device fd files

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Sep 01, 2011 at 03:26:01PM -0500, Scott Wood wrote:
> On 09/01/2011 03:00 PM, Michael S. Tsirkin wrote:
> > That's a very rich interface, and easy to get wrong.
> > AFAIK the only reason vfio uses read/write for PCI was to avoid inventing
> > a custom interface. But if you do, it looks like a set of ioctls would
> > be much easier? You can even fit the existing uio infrastructure if you like.
> 
> How would it be easier than producing/parsing a static data structure?
> What would it look like?

For example, for a property X, instead of adding a structure
with identifier X, implement ioctl GET_X. Userspace
calls this ioctl, an error implies the property
is not present.


> > Here's another idea:  all the information is likely already available
> > in sysfs.
> 
> The only major thing that is likely available elsewhere is PCI config
> space, and that was not new to this proposal.
> Most other material is specifically related to the vfio/dtio interface
> (e.g. offsets into the file handle, arguments to the "get irq fd" ioctl,
> mapping of dtio regions/interrupts to device tree nodes), and could not
> be "useful to more than just vfio".

For example resources are already there in sysfs.

> > A way to query where the device is in sysfs
> > would give you *a ton* of information, including the type etc,
> 
> For PCI, the user has domain/bus/dev/fn which should be sufficient to
> find that, if desired.  For device-tree devices, there's a device tree
> path provided for each region/interrupt.
> 
> -Scott

So you are saying the user already has sysfs path?
I thought the point was to pass all info through
a single fd.