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

Leon Romanovsky
2026-04-09T12:04:22Z

On Tue, Mar 31, 2026 at 01:44:02PM -0600, Keith Busch wrote:
> On Tue, Mar 31, 2026 at 10:02:20PM +0300, Leon Romanovsky wrote:
> >
> > Right, what about adding TPH fields to struct vfio_region_dma_range
> > instead of struct vfio_device_feature_dma_buf?
>
> You might have to show me with code what you're talking about because I
> can't see any way we can add fields to any struct here without breaking
> backward compatibility.
>
> If we can't claim bits out of the unused "flags" field for this feature,
> then my initial reply is the only sane approach: we can introduce a new
> feature and struct for it that closely mirrors the existing one, but
> with the extra hint fields.

Something like that, on top of this proposal:

diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c
index 3961afa640391..70d5ee1e3ef7b 100644
--- a/drivers/vfio/pci/vfio_pci_dmabuf.c
+++ b/drivers/vfio/pci/vfio_pci_dmabuf.c
@@ -241,9 +241,7 @@ int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
return -EFAULT;

if (!get_dma_buf.nr_ranges ||
- (get_dma_buf.flags & ~(VFIO_DMABUF_FL_TPH |
- VFIO_DMABUF_TPH_PH_MASK |
- VFIO_DMABUF_TPH_ST_MASK)))
+ (get_dma_buf.flags & ~VFIO_DMABUF_FLAG_TPH))
return -EINVAL;

/*
@@ -300,13 +298,10 @@ int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
ret = PTR_ERR(priv->dmabuf);
goto err_dev_put;
}
- if (get_dma_buf.flags & VFIO_DMABUF_FL_TPH) {
- priv->steering_tag = (get_dma_buf.flags &
- VFIO_DMABUF_TPH_ST_MASK) >>
- VFIO_DMABUF_TPH_ST_SHIFT;
- priv->ph = (get_dma_buf.flags &
- VFIO_DMABUF_TPH_PH_MASK) >>
- VFIO_DMABUF_TPH_PH_SHIFT;
+ if (get_dma_buf.flags & VFIO_DMABUF_FLAG_TPH) {
+ priv->steering_tag =
+ dma_ranges[get_dma_buf.nr_ranges + 1].tph.tag;
+ priv->ph = dma_ranges[get_dma_buf.nr_ranges + 1].tph.ph;
}
/* dma_buf_put() now frees priv */
INIT_LIST_HEAD(&priv->dmabufs_elm);
diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
index e2a8962641d2c..a8b8d8b1a3278 100644
--- a/include/uapi/linux/vfio.h
+++ b/include/uapi/linux/vfio.h
@@ -1497,20 +1497,30 @@ struct vfio_device_feature_bus_master {
*/
#define VFIO_DEVICE_FEATURE_DMA_BUF 11

+struct vfio_region_dma_tph {
+ u16 tag;
+ u8 ph;
+};
+
struct vfio_region_dma_range {
- __u64 offset;
- __u64 length;
+ union {
+ __u64 offset;
+ struct vfio_region_dma_tph tph;
+ };
+ union {
+ __u64 length;
+ __u64 reserved;
+ };
+};
+
+enum {
+ VFIO_DMABUF_FLAG_TPH = 1 << 0,
};

struct vfio_device_feature_dma_buf {
__u32 region_index;
__u32 open_flags;
__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
__u32 nr_ranges;
struct vfio_region_dma_range dma_ranges[] __counted_by(nr_ranges);
};