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

Leon Romanovsky
2026-03-25T08:49:33Z

On Tue, Mar 24, 2026 at 04:46:02PM -0700, Zhiping Zhang wrote:
> This patch adds a callback to get the tph info on DMA buffer exporters.
> The tph info includes both the steering tag and the process hint (ph).
>
> The steering tag and ph are encoded in the flags field of
> vfio_device_feature_dma_buf instead of adding new fields to the uapi
> struct, to preserve ABI compatibility.
>
> Signed-off-by: Zhiping Zhang <zhipingz@meta.com>
> ---
> drivers/vfio/pci/vfio_pci_dmabuf.c | 26 ++++++++++++++++++++++++--
> include/linux/dma-buf.h | 30 ++++++++++++++++++++++++++++++
> include/uapi/linux/vfio.h | 9 +++++++--
> 3 files changed, 61 insertions(+), 4 deletions(-)

<...>

> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> index bb7b89330d35..e2a8962641d2 100644
> --- a/include/uapi/linux/vfio.h
> +++ b/include/uapi/linux/vfio.h
> @@ -1505,8 +1505,13 @@ struct vfio_region_dma_range {
> 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.

> struct vfio_region_dma_range dma_ranges[] __counted_by(nr_ranges);
> };
>
> --
> 2.52.0
>
>