[PATCH v13 5/7] vfio: ABI for mdev display dma-buf operation

[Gerd Hoffmann]

  Hi,

> These are both from Gerd.  Gerd, do you have any objection to using a
> union to provide either the dmabuf fd or region index?

No.

> > It's like we want to propose a general interface used to share
> > guest's buffer with host. And the
> > general interface, so far, has two choice: region and dma-buf. So
> > each mdev likes this interface
> > can implement one kind of it and gets the benefit from the general
> > interface.
> > So, if we think about this, the difference in user mode should be
> > as little as possible.
> 
> The difference seems pretty minimal here, the user probes supported
> interface types, and explicitly picks one by requesting updates using
> that interface type.  The difference is only in the interpretation of
> one dword field.  Furthermore, we're not limiting ourselves to these
> two interface types, this same API could support dmabuf-v2 if we
> define
> a flag bit for it and define the structure of the interface union.

Yep, using the flags is more future-proof and continues to work in case
another interface type shows up (unlike looking for a gfx region being
present).

> > Agree, that's a good proposal, which can handle all the cases.
> > I'm just not sure about the usage case of "on every call". In
> > previous discussion, it seems we think static is enough.
> 
> Is it somehow a problem for the user to set the type bit in the flag
> on
> every call?

Not at all.

cheers,
  Gerd