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

From: Leon Romanovsky <[email-masked]>
Date: Tue, 31 Mar 2026 11:37:58 +0300

On Thu, Mar 26, 2026 at 04:41:11PM -0600, Keith Busch 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
> > }
> > 
> > > +	__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.

He needs to add before flex array. This struct is submitted by the user
and kernel can easily calculate the position of that array.

Something like this:
diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c
index b1d658b8f7b51..d78d915992232 100644
--- a/drivers/vfio/pci/vfio_pci_dmabuf.c
+++ b/drivers/vfio/pci/vfio_pci_dmabuf.c
@@ -237,7 +237,11 @@ int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
        if (get_dma_buf.region_index >= VFIO_PCI_ROM_REGION_INDEX)
                return -ENODEV;

-       dma_ranges = memdup_array_user(&arg->dma_ranges, get_dma_buf.nr_ranges,
+       if (!tph_supplied)
+               dma_ranges = memdup_array_user(old_dma_ranges_pos, get_dma_buf.nr_ranges,
+                                      sizeof(*dma_ranges));
+       else
+               dma_ranges = memdup_array_user(&arg->dma_ranges, get_dma_buf.nr_ranges,
                                       sizeof(*dma_ranges));
        if (IS_ERR(dma_ranges))
                return PTR_ERR(dma_ranges);
~

Thanks

> 
> 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);
> > >  };