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

From: Leon Romanovsky <[email-masked]>
Date: Thu, 9 Apr 2026 15:04:15 +0300

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);
 };