Re: [PATCH v14 5/7] vfio: ABI for mdev display dma-buf operation

[Gerd Hoffmann <kraxel@redhat.com>]

  Hi,

[ bringing a private discussion back to the list ]

> The dma-buf's life cycle is handled by user mode and tracked by
> kernel.
> The returned fd in struct vfio_device_query_gfx_plane can be a new
> fd or an old fd of a re-exported dma-buf. Host user mode can check
> the
> value of fd and to see if it needs to create new resource according
> to
> the new fd or just use the existed resource related to the old fd.

Ok, this idea has a fundamental flaw:  The life cycle of the dma-buf
and the file handle are not identical.  The dma-buf can exist longer
than the file handle in case other references to the dma-buf exist.  So
when trying to use the file handle as identifier for the dma-buf you'll
end up with all sorts of strange effects.

So, I'd suggest to use a id instead, and add a ioctl to get a dmabuf
for a given id (incremental patch):

--- a/include/uapi/linux/vfio.h
+++ b/include/uapi/linux/vfio.h
@@ -538,12 +538,22 @@ struct vfio_device_gfx_plane_info {
        __u32 y_pos;    /* vertical position of cursor plane, upper
left corner in lines*/
        union {
                __u32 region_index;     /* region index */
-               __s32 fd;       /* dma-buf fd */
+               __u32 dmabuf_id;        /* dma-buf id */
        };
 };
 
 #define VFIO_DEVICE_QUERY_GFX_PLANE _IO(VFIO_TYPE, VFIO_BASE + 14)
 
+struct vfio_device_gfx_dmabuf_fd {
+       __u32 argsz;
+       __u32 flags;
+       /* in */
+       __u32 dmabuf_id;
+       /* out */
+       __s32 dmabuf_fd;
+};
+
+#define VFIO_DEVICE_GET_GFX_DMABUF _IO(VFIO_TYPE, VFIO_BASE + 15)
 
 /* -------- API for Type1 VFIO IOMMU -------- */

[ no changes for a region-based display ]

git branch, kernel, with updated dmabuf patch:
  https://www.kraxel.org/cgit/linux/log/?h=gvt-dmabuf-v14

qemu branch:
  https://www.kraxel.org/cgit/qemu/log/?h=work/intel-vgpu

cheers,
  Gerd

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