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

Zhiping Zhang
2026-03-26T22:55:57Z

On Thu, Mar 26, 2026 at 3:41 PM Keith Busch <kbusch@kernel.org> wrote:
>
> >
> 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
> > }

yes we can do that.

> >
> > > + __u32 nr_ranges;
> >
> > add your "__u16 steering_tag" and "__u8 ph" fields here.
>
That is what I did in V1, Leon.

> 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.
>
if not using the fields in the flag, then we probably have to
introduce a new vfio
device feature.

> > > struct vfio_region_dma_range dma_ranges[] __counted_by(nr_ranges);
> > > };