Re: [PATCH v9 5/7] vfio: Define vfio based dma-buf operations

[Gerd Hoffmann <kraxel@redhat.com>]

  Hi,

> > With the generation we can also do something different:  Pass in
> > plane_type and
> > generation, and have VFIO_DEVICE_GET_DMABUF_FD return an error in
> > case
> > the generation doesn't match.  In that case it doesn't make much
> > sense any
> > more to have a separate plane_info struct, which was added so we
> > don't have
> > to duplicate things in query-plane and get- dmabuf ioctl structs.
> 
> Comparing with the current patch, this would make user space a little
> bit harder to
> get the dmabuf by calling VFIO_DEVICE_GET_DMABUF ioctl. Is it
> efficient for
> user mode usage?

user space has to call QUERY-PLANE first, then looks if it has a dma-
buf for that, if not call GET-DMABUF.

Problem is the guest could have changed the plane between the QUERY-
PLANE and GET-DMABUF ioctls.

Current patches (v8 series) just returns plane-info on GET-DMABUF too,
so userspace can at least detect something changed.

It would be easier for userspace if GET-DMABUF throws an error in case
the plane changed since the last QUERY-PLANE ioctl.  The generation id
would be one way to handle it, but possibly it is easier if the kernel
driver just keeps track internally.  So GET-DMABUF would be defined to
return a dmabuf for the plane returned by the previous QUERY-PLANE
ioctl (on the same file handle), or return an error in case the plane
has changed meanwhile.

cheers,
  Gerd

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