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

[Alex Williamson <alex.williamson@redhat.com>]

On Tue, 20 Jun 2017 23:01:53 +0000
"Zhang, Tina" <tina.zhang@intel.com> wrote:

> > -----Original Message-----
> > From: Alex Williamson [mailto:alex.williamson@redhat.com]
> > Sent: Tuesday, June 20, 2017 11:00 PM
> > To: Gerd Hoffmann <kraxel@redhat.com>
> > Cc: Zhang, Tina <tina.zhang@intel.com>; intel-gfx@lists.freedesktop.org; linux-
> > kernel@vger.kernel.org; Kirti Wankhede <kwankhede@nvidia.com>; Chen,
> > Xiaoguang <xiaoguang.chen@intel.com>; intel-gvt-dev@lists.freedesktop.org;
> > Lv, Zhiyuan <zhiyuan.lv@intel.com>; Wang, Zhi A <zhi.a.wang@intel.com>;
> > Wang, Zhenyu Z <zhenyu.z.wang@intel.com>
> > 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@redhat.com> 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.

In this v9 series, we have a VFIO_DEVICE_GET_FD where we could pass
VFIO_DEVICE_DMABUF_MGR_FD to get the manager fd, from that fd we could
query plane info or get a dmabuf fd.  Now we're getting rid of the
manager fd and I'm questioning whether generic ioctls or specific
ioctls are the right path.

For instance we could still use a VFIO_DEVICE_GET_FD ioctl to get the
dmabuf fd rather than creating a VFIO_DEVICE_GET_DMABUF_FD ioctl.  It's
just a matter of defining a common header on the data structure so that
we know how to interpret the remainder of the structure.

Likewise Gerd proposes above a VFIO_DEVICE_QUERY_GFX_PLANE ioctl and
I'm asking whether a generic VFIO_DEVICE_QUERY ioctl where we define a
common header in the structure to know that the query is for graphics
plane information has value.

IOW, should we spend a little bit of time now crafting ioctls that we
can use for purposes beyond what we're looking at today, or do we burn
off a couple for singular uses here?  Thanks,

Alex


> > > 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  

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