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

Keith Busch
2026-03-26T22:41:14Z

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);
> > };