RE: [Intel-gfx] [PATCH v9 5/7] vfio: Define vfio based dma-buf operations

[Zhang, Tina]

> -----Original Message-----
> From: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx]
> Sent: Tuesday, June 20, 2017 11:00 PM
> To: Gerd Hoffmann <kraxel@xxxxxxxxxx>
> Cc: Zhang, Tina <tina.zhang@xxxxxxxxx>; intel-gfx@xxxxxxxxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; Kirti Wankhede <kwankhede@xxxxxxxxxx>; Chen,
> Xiaoguang <xiaoguang.chen@xxxxxxxxx>; intel-gvt-dev@xxxxxxxxxxxxxxxxxxxxx;
> Lv, Zhiyuan <zhiyuan.lv@xxxxxxxxx>; Wang, Zhi A <zhi.a.wang@xxxxxxxxx>;
> Wang, Zhenyu Z <zhenyu.z.wang@xxxxxxxxx>
> Subject: Re: [Intel-gfx] [PATCH v9 5/7] vfio: Define vfio based dma-buf
> operations
> 
> On Tue, 20 Jun 2017 12:57:36 +0200
> Gerd Hoffmann <kraxel@xxxxxxxxxx> wrote:
> 
> > On Tue, 2017-06-20 at 08:41 +0000, Zhang, Tina wrote:
> > > Hi,
> > >
> > > Thanks for all the comments. Here are the summaries:
> > >
> > > 1. Modify the structures to make it more general.
> > > struct vfio_device_gfx_plane_info {
> > > 	__u64 start;
> > > 	__u64 drm_format_mod;
> > > 	__u32 drm_format;
> > > 	__u32 width;
> > > 	__u32 height;
> > > 	__u32 stride;
> > > 	__u32 size;
> > > 	__u32 x_pos;
> > > 	__u32 y_pos;
> > > 	__u32 generation;
> > > };
> >
> > Looks good to me.
> >
> > > struct vfio_device_query_gfx_plane {
> > > 	__u32 argsz;
> > > 	__u32 flags;
> > > #define VFIO_GFX_PLANE_FLAGS_REGION_ID		(1 << 0)
> > > #define VFIO_GFX_PLANE_FLAGS_PLANE_ID		(1 << 1)
> > > 	struct vfio_device_gfx_plane_info plane_info;
> > > 	__u32 id;
> > > };
> >
> > I'm not convinced the flags are a great idea.  Whenever dmabufs or a
> > region is used is a static property of the device, not of each
> > individual plane.
> >
> >
> > I think we should have this for userspace to figure:
> >
> > enum vfio_device_gfx_type {
> >         VFIO_DEVICE_GFX_NONE,
> >         VFIO_DEVICE_GFX_DMABUF,
> >         VFIO_DEVICE_GFX_REGION,
> > };
> >
> > struct vfio_device_gfx_query_caps {
> >         __u32 argsz;
> >         __u32 flags;
> >         enum vfio_device_gfx_type;
> > };
> 
> We already have VFIO_DEVICE_GET_INFO which returns:
> 
> struct vfio_device_info {
>         __u32   argsz;
>         __u32   flags;
> #define VFIO_DEVICE_FLAGS_RESET (1 << 0)        /* Device supports reset */
> #define VFIO_DEVICE_FLAGS_PCI   (1 << 1)        /* vfio-pci device */
> #define VFIO_DEVICE_FLAGS_PLATFORM (1 << 2)     /* vfio-platform device */
> #define VFIO_DEVICE_FLAGS_AMBA  (1 << 3)        /* vfio-amba device */
> #define VFIO_DEVICE_FLAGS_CCW   (1 << 4)        /* vfio-ccw device */
>         __u32   num_regions;    /* Max region index + 1 */
>         __u32   num_irqs;       /* Max IRQ index + 1 */
> };
> 
> We could use two flag bits to indicate dmabuf or graphics region support.
> vfio_device_gfx_query_caps seems to imply a new ioctl, which would be
> unnecessary.
> 
> > Then this to query the plane:
> >
> > struct vfio_device_gfx_query_plane {
> >         __u32 argsz;
> >         __u32 flags;
> >         struct vfio_device_gfx_plane_info plane_info;  /* out */
> >         __u32 plane_type;                              /* in  */
> > };
> 
> I'm not sure why we're using an enum for something that can currently be
> defined with 2 bits, seems like this would be another good use of flags.  We
> could even embed an enum into the flags if we want to leave some expansion
> room, 4 bits maybe?  Also, I was imagining that a device could support multiple
> graphics regions, that's where specifying the "id" as a region index seemed
> useful.  We lose that ability here unless we go back to defining a flag bit to
> specify how to interpret this last field.
> 
> > 2. Remove dmabuf mgr fd and add these two ioctl commands to the vfio
> > device fd.
> > > VFIO_DEVICE_QUERY_GFX_PLANE : used to query
> > > vfio_device_gfx_plane_info.
> >
> > Yes.
> >
> > > VFIO_DEVICE_GET_DMABUF_FD: used to create and return the dmabuf fd.
> 
> I'm not convinced this adds value, but I'll list it as an option:
> 
> VFIO_DEVICE_QUERY(VFIO_DEVICE_GFX_PLANE)
> VFIO_DEVICE_GET_FD(VFIO_DEVICE_GFX_DMABUF_FD)
> 
> The benefit is that it might help to avoid a proliferation of ioctls on the device the
> pain is that we need to either define a field or section of flags which identify
> what is being queried or what type of device fd is being requested.
I didn't understand here. The patch introduces three ioctl commands: VFIO_DEVICE_GET_FD, VFIO_DMABUF_MGR_QUERY_PLANE, VFIO_DMABUF_MGR_CREATE_DMABUF.
What I mean was we could remove the first one, a.k.a  VFIO_DEVICE_GET_FD, which is used to get the fd of dmabuf mgr, as we want to remove the logic of dmabuf mgr. For the other two ioctls, I think we can give them new names which looks like more general. 
So, do you mean there is another way instead of ioctls? Thanks.

BR,
Tina

> > Yes.  The plane might have changed between query-plane and get-dmabuf
> > ioctl calls though, we must make sure we handle that somehow.  Current
> > patches return plane_info on get-dmabuf ioctl too, so userspace can
> > see what it actually got.
> >
> > 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.
> 
> I'm not sure I understand how this works for a region, the region is always the
> current generation, how can the user ever be sure the plane_info matches what
> is exposed in the region?  Thanks,
> 
> Alex