RE: [PATCH v15 5/7] vfio: ABI for mdev display dma-buf operation

[Zhang, Tina]

> -----Original Message-----
> From: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx]
> Sent: Wednesday, October 11, 2017 3:17 AM
> To: Zhang, Tina <tina.zhang@xxxxxxxxx>
> Cc: kraxel@xxxxxxxxxx; chris@xxxxxxxxxxxxxxxxxx; zhenyuw@xxxxxxxxxxxxxxx;
> Lv, Zhiyuan <zhiyuan.lv@xxxxxxxxx>; Wang, Zhi A <zhi.a.wang@xxxxxxxxx>;
> Tian, Kevin <kevin.tian@xxxxxxxxx>; daniel@xxxxxxxx; kwankhede@xxxxxxxxxx;
> intel-gfx@xxxxxxxxxxxxxxxxxxxxx; intel-gvt-dev@xxxxxxxxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; Daniel Vetter <daniel.vetter@xxxxxxxx>
> Subject: Re: [PATCH v15 5/7] vfio: ABI for mdev display dma-buf operation
> 
> On Tue, 10 Oct 2017 17:50:05 +0800
> Tina Zhang <tina.zhang@xxxxxxxxx> wrote:
> 
> > Add VFIO_DEVICE_QUERY_GFX_PLANE ioctl command to let user mode query
> > and get the plan and its related information. This ioctl can be invoked with:
> 
> s/plan/plane/
Sorry about this typo :).

> 
> > 1) either flag DMABUF or REGION is set. Vendor driver returns success
> > and the plane_info only when the specific kind of buffer is supported.
> > 2) flag PROBE is set with either DMABUF or REGION. Vendor driver
> > returns success only when the specific kind of buffer is supported.
> >
> > 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.
> >
> > v15:
> > - add a ioctl to get a dmabuf for a given dmabuf id. (Gerd)
> >
> > v14:
> > - add PROBE, DMABUF and REGION flags. (Alex)
> >
> > v12:
> > - add drm_format_mod back. (Gerd and Zhenyu)
> > - add region_index. (Gerd)
> >
> > v11:
> > - rename plane_type to drm_plane_type. (Gerd)
> > - move fields of vfio_device_query_gfx_plane to vfio_device_gfx_plane_info.
> >   (Gerd)
> > - remove drm_format_mod, start fields. (Daniel)
> > - remove plane_id.
> >
> > v10:
> > - refine the ABI API VFIO_DEVICE_QUERY_GFX_PLANE. (Alex) (Gerd)
> >
> > v3:
> > - add a field gvt_plane_info in the drm_i915_gem_obj structure to save
> >   the decoded plane information to avoid look up while need the plane
> >   info. (Gerd)
> >
> > Signed-off-by: Tina Zhang <tina.zhang@xxxxxxxxx>
> > Cc: Gerd Hoffmann <kraxel@xxxxxxxxxx>
> > Cc: Alex Williamson <alex.williamson@xxxxxxxxxx>
> > Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> > ---
> >  include/uapi/linux/vfio.h | 62
> > +++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 62 insertions(+)
> >
> > diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> > index ae46105..fdf9a9c 100644
> > --- a/include/uapi/linux/vfio.h
> > +++ b/include/uapi/linux/vfio.h
> > @@ -502,6 +502,68 @@ struct vfio_pci_hot_reset {
> >
> >  #define VFIO_DEVICE_PCI_HOT_RESET	_IO(VFIO_TYPE, VFIO_BASE +
> 13)
> >
> > +/**
> > + * VFIO_DEVICE_QUERY_GFX_PLANE - _IOW(VFIO_TYPE, VFIO_BASE + 14,
> > + *                                    struct vfio_device_query_gfx_plane)
> > + *
> > + * Set the drm_plane_type and flags, then retrieve the gfx plane info.
> > + *
> > + * flags supported:
> > + * - VFIO_GFX_PLANE_TYPE_PROBE and VFIO_GFX_PLANE_TYPE_DMABUF
> are set
> > + *   to ask if the mdev supports dma-buf. 0 on support, -EINVAL on no
> > + *   support for dma-buf.
> > + * - VFIO_GFX_PLANE_TYPE_PROBE and VFIO_GFX_PLANE_TYPE_REGION
> are set
> > + *   to ask if the mdev supports region. 0 on support, -EINVAL on no
> > + *   support for region.
> > + * - VFIO_GFX_PLANE_TYPE_DMABUF or VFIO_GFX_PLANE_TYPE_REGION
> is set
> > + *   with each call to query the plane info.
> 
> So dmabuf_id is effectively just a token that can be fed into GET_GFX_DMABUF
> to get the fd.  The implementation of the token is vendor specific, but can be
> thought of as some sort of sequence ID or generation ID (but not necessarily
> monotonically increasing), so GET_GFX_DMABUF may fail if the previously
> provided dmabuf_id is no longer valid.  Do I have this correct?
Exactly. GET_GFX_DMABUF may fail if the dmabuf_id is no longer valid. -EINVAL will be
returned in that case. 
And dmabuf_id is invalid when:
1) user space has closed all dma-buf fds which are exposed with the dmabuf_id,
2) or the dmabuf_id isn't returned from QUERY_GFX_PLANE.
I will add the comments. Thanks.

> 
> > + * - Others are invalid and return -EINVAL.
> 
> And I see that in patch 7/7 that i915 is checking explicitly for only these flag
> combinations, great!
> 
> > + *
> > + * Return: 0 on success, -ENODEV with all out fields zero on mdev
> > + * device initialization, -errno on other failure.
> > + */
> > +struct vfio_device_gfx_plane_info {
> > +	__u32 argsz;
> > +	__u32 flags;
> > +#define VFIO_GFX_PLANE_TYPE_PROBE (1 << 0) #define
> > +VFIO_GFX_PLANE_TYPE_DMABUF (1 << 1) #define
> > +VFIO_GFX_PLANE_TYPE_REGION (1 << 2)
> > +	/* in */
> > +	__u32 drm_plane_type;	/* type of plane: DRM_PLANE_TYPE_* */
> > +	/* out */
> > +	__u32 drm_format;	/* drm format of plane */
> > +	__u64 drm_format_mod;   /* tiled mode */
> > +	__u32 width;	/* width of plane */
> > +	__u32 height;	/* height of plane */
> > +	__u32 stride;	/* stride of plane */
> > +	__u32 size;	/* size of plane in bytes, align on page*/
> > +	__u32 x_pos;	/* horizontal position of cursor plane */
> > +	__u32 y_pos;	/* vertical position of cursor plane*/
> > +	union {
> > +		__u32 region_index;	/* region index */
> > +		__s32 dmabuf_id;	/* dma-buf fd */
> 
> "dma-buf fd", but it's not an fd.  Why is this signed since it's no longer an fd?
Again, sorry about the typo :)

> 
> > +	};
> > +};
> > +
> > +#define VFIO_DEVICE_QUERY_GFX_PLANE _IO(VFIO_TYPE, VFIO_BASE + 14)
> > +
> > +/**
> > + * VFIO_DEVICE_GET_GFX_DMABUF - _IOW(VFIO_TYPE, VFIO_BASE + 15,
> > + *				    struct vfio_device_gfx_dmabuf_fd)
> > + *
> > + * Return: 0 on success, -errno on failure.
> > + */
> 
> So given a dmabuf_id, return a dmabuf_fd, which may be the same as a fd
> previously returned to the user.  In the latter case, can we assume that there's
> no new reference added to the existing fd?  Is the user permitted to cache the
> dmabuf_id to dmabuf_fd mapping?  Similarly, if a user already has a dmabuf_fd,
> can calling GET_GFX_DMABUF ever fail or return a different result for the same
> dmabuf_id which provided that fd?  Maybe we could define some of these
> behaviors in the header file to keep vendor implementations consistent.

Actually, a new fd is returned with each time GET_GFX_DMABUF invoked, even
with the same dmabuf_id. The benefit is to leave the dma-buf's life cycle management
to userspace. The dma-buf will be released when all its fds in userspace are closed.

Then, I think I need to add some comments like this:
"dmabuf_fd will return a new fd with each time GET_GFX_DMABUF being invoked"


> 
> > +struct vfio_device_gfx_dmabuf_fd {
> > +	__u32 argsz;
> > +	__u32 flags;
> 
> Patch 7/7 however fails to validate this flags field.  It needs to return -EINVAL if
> any unsupported flags are set, which at this point is all of them.
Indeed.
 
> 
> > +	/* in */
> > +	__u32 dmabuf_id;
> 
> This unsigned dmabuf_id is probably correct, but it doesn't match the signed
> one provided above.
Will make them both unsigned :)

> 
> > +	/* out */
> > +	__s32 dmabuf_fd;
> > +};
> 
> VFIO_GROUP_GET_DEVICE_FD returns the fd through the return code, is there
> a reason to do it differently here?  GET_DEVICE_FD also only takes a char
> parameter without the extra future proofing, I wonder if this ioctl is simple
> enough to do the same.
So that means we don't need the "flages" field and change dmabuf_id to a character string.
Maybe it's fine for the current usage. But, I'm not sure if it's suitable for future.
Gerd, Zhenyu and Zhi, how do you think about it?

Thanks.
Tina
> 
> > +
> > +#define VFIO_DEVICE_GET_GFX_DMABUF _IO(VFIO_TYPE, VFIO_BASE + 15)
> > +
> >  /* -------- API for Type1 VFIO IOMMU -------- */
> >
> >  /**
> 
> Thanks,
> Alex