Re: [RFC v2 1/2] vfio: add callback to get tph info for dmabuf

[Keith Busch <kbusch@kernel.org>]

On Wed, Mar 25, 2026 at 10:25:34AM +0200, Leon Romanovsky wrote:
> On Tue, Mar 24, 2026 at 04:46:02PM -0700, Zhiping Zhang wrote:
> >  struct vfio_device_feature_dma_buf {
> >  	__u32	region_index;
> >  	__u32	open_flags;
> > -	__u32   flags;
> > -	__u32   nr_ranges;
> > +	__u32	flags;
> > +#define VFIO_DMABUF_FL_TPH		(1U << 0) /* TPH info is present */
> > +#define VFIO_DMABUF_TPH_PH_SHIFT	1         /* bits 1-2: PH (2-bit) */
> > +#define VFIO_DMABUF_TPH_PH_MASK	0x6U
> > +#define VFIO_DMABUF_TPH_ST_SHIFT	16        /* bits 16-31: steering tag */
> > +#define VFIO_DMABUF_TPH_ST_MASK		0xffff0000U
> 
> This extension of flags is basically kills future extension of this
> struct for anything that includes TPH.
> 
> Add new
> enum vfio_device_feature_dma_buf_flags {
>     VFIO_DMABUF_FL_TPH  = 1 << 0
> }
> 
> > +	__u32	nr_ranges;
> 
> add your "__u16 steering_tag" and "__u8 ph" fields here.

You're suggesting that Ziping append the new fields to the end of this
struct? I don't think we can modify the layout of a uapi.

If we can't carve the space for this out of the existing unused flags
field, I think we'd have to introduce a new vfio device feature that
basically copies VFIO_DEVICE_FEATURE_DMA_BUF with the extra hints
fields.
 
> >  	struct vfio_region_dma_range dma_ranges[] __counted_by(nr_ranges);
> >  };