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

[Leon Romanovsky <leon@kernel.org>]

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
> 
>