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

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

[Leon Romanovsky]

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
> 
> 

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

[Keith Busch]

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);
> >  };

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

[Zhiping Zhang]

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);
> > >  };