[Linaro-mm-sig] Re: [PATCH 3/9] vfio/pci: Add a helper to create a DMABUF for a BAR-map VMA

On Thu, Apr 16, 2026 at 06:17:46AM -0700, Matt Evans wrote:

+int vfio_pci_core_mmap_prep_dmabuf(struct vfio_pci_core_device *vdev,

• 		   struct vm_area_struct *vma,
  

• 		   u64 phys_start, u64 req_len,
  

• 		   unsigned int res_index)
  

+{

• struct vfio_pci_dma_buf *priv;
• const unsigned int nr_ranges = 1;
• int ret;
• priv = kzalloc_obj(*priv);
• if (!priv)

• return -ENOMEM;
  

• priv->phys_vec = kzalloc_obj(*priv->phys_vec);
• if (!priv->phys_vec) {

• ret = -ENOMEM;
  

• goto err_free_priv;
  

• }
• /*

• * The mmap() request's vma->vm_offs might be non-zero, but
  

• * the DMABUF is created from _offset zero_ of the BAR.  The
  

• * portion between zero and the vm_offs is inaccessible
  

• * through this VMA, but this approach keeps the
  

• * /proc/<pid>/maps offset somewhat consistent with the
  

• * pre-DMABUF code.  Size includes the offset portion.
  

I'm not sure I understand this comment?

For the old path vm_pgoff for byte 0 of the bar starts at some large offset

For the new path vm_pgoff for byte 0 of the first range starts at 0

• * This differs from an mmap() of an explicitly-exported
  

• * DMABUF which is an arbitrary slice of the BAR, would be
  

• * created with the desired offset+size, and would usually be
  

• * mmap()ed with pgoff = 0.
  

• *
  

• * Both are equivalent and vfio_pci_dma_buf_find_pfn() finds
  

• * the same PFNs.
  

• */
  

• priv->vdev = vdev;
• priv->nr_ranges = nr_ranges;
• priv->size = (vma->vm_pgoff << PAGE_SHIFT) + req_len;

And why is size being calculated from pgoff ?

Jason