Re: [PATCH v8 4/9] vfio/pci: Add a helper to look up PFNs for DMABUFs

[Alex Williamson <alex@shazbot.org>]

On Tue,  6 Oct 2026 20:36:32 +0100
Matt Evans <matt@ozlabs.org> wrote:

> Add vfio_pci_dma_buf_find_pfn(), which a VMA fault handler can use to
> find a PFN.
> 
> This supports multi-range DMABUFs, which typically would be used to
> represent scattered spans but might even represent overlapping or
> aliasing spans of PFNs.
> 
> Because this is intended to be used in vfio_pci_core.c, we also need
> to expose the struct vfio_pci_dma_buf in the vfio_pci_priv.h header.
> 
> This move also adds a new member, vma_pgoff_adjust; this is consumed
> by vfio_pci_dma_buf_find_pfn(), but will be set in a future commit.
> Add it now to reduce the number of functions touched by later commits.
> 
> Signed-off-by: Matt Evans <matt@ozlabs.org>
> ---
>  drivers/vfio/pci/vfio_pci_dmabuf.c | 166 ++++++++++++++++++++++++++---
>  drivers/vfio/pci/vfio_pci_priv.h   |  21 ++++
>  2 files changed, 174 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c
> index 847ee5cc78e7..9f10b10fc436 100644
> --- a/drivers/vfio/pci/vfio_pci_dmabuf.c
> +++ b/drivers/vfio/pci/vfio_pci_dmabuf.c
> @@ -9,19 +9,6 @@
>  
>  MODULE_IMPORT_NS("DMA_BUF");
>  
> -struct vfio_pci_dma_buf {
> -	struct dma_buf *dmabuf;
> -	struct vfio_pci_core_device *vdev;
> -	struct list_head dmabufs_elm;
> -	size_t size;
> -	struct phys_vec *phys_vec;
> -	struct p2pdma_provider *provider;
> -	u32 nr_ranges;
> -	struct kref kref;
> -	struct completion comp;
> -	u8 revoked : 1;
> -};
> -
>  static int vfio_pci_dma_buf_attach(struct dma_buf *dmabuf,
>  				   struct dma_buf_attachment *attachment)
>  {
> @@ -106,6 +93,159 @@ static const struct dma_buf_ops vfio_pci_dmabuf_ops = {
>  	.release = vfio_pci_dma_buf_release,
>  };
>  
> +int vfio_pci_dma_buf_find_pfn(struct vfio_pci_core_device *vdev,
> +			      struct vfio_pci_dma_buf *priv,
> +			      struct vm_area_struct *vma,
> +			      unsigned long fault_addr,
> +			      unsigned int order,
> +			      unsigned long *out_pfn)
> +{
> +	/*
> +	 * Given a VMA (start, end, pgoffs) and a fault address,
> +	 * search the corresponding DMABUF's phys_vec[] to find the
> +	 * range representing the address's offset into the VMA, and
> +	 * its PFN.  vdev must be the device that the DMABUF priv was
> +	 * exported from; vdev->dmabuf_lock must be held, and priv
> +	 * must not be revoked.
> +	 *
> +	 * The phys_vec[] ranges represent contiguous spans of VAs
> +	 * upwards from the buffer offset 0; the actual PFNs might be
> +	 * in any order, overlap/alias, etc.  Calculate an offset of
> +	 * the desired page given VMA start/pgoff and address, then
> +	 * search upwards from 0 to find which span contains it.
> +	 *
> +	 * On success, a valid PFN for a page sized by 'order' is
> +	 * returned into out_pfn.
> +	 *
> +	 * Failure occurs if:
> +	 * - A hugepage would cross the edge of the VMA,
> +	 * - A hugepage isn't entirely contained within a range
> +	 *   (including where it straddles the boundary between
> +	 *   ranges),
> +	 * - We find a range, but the final PFN isn't aligned to the
> +	 *   requested order.
> +	 *
> +	 * Upon failure, -ERANGE is returned and the caller is
> +	 * expected to try again with a smaller order, which will
> +	 * eventually succeed.
> +	 *
> +	 * It's suboptimal if DMABUFs are created with neighbouring
> +	 * ranges that are physically contiguous, since hugepages
> +	 * can't straddle range boundaries.  (The construction of the
> +	 * ranges should merge them in this case.)
> +	 *
> +	 * Finally, vma_pgoff_adjust is used with a DMABUF created for
> +	 * a VFIO BAR mmap: a BAR mapped with vm_pgoff > 0 creates a
> +	 * DMABUF such that byte 0 of the VMA corresponds to byte 0 of
> +	 * the DMABUF and byte 'vm_pgoff << PAGE_SHIFT' into the BAR.
> +	 * To avoid double-offsetting in this scenario, subtracting
> +	 * vma_pgoff_adjust from this (non-zero) vm_pgoff generates
> +	 * the effective offset.  This also removes the VFIO region
> +	 * index encoded in vm_pgoff for VFIO BAR mmaps.
> +	 */
> +
> +	const unsigned long pagesize = PAGE_SIZE << order;
> +	unsigned long vma_off = (vma->vm_pgoff - priv->vma_pgoff_adjust) <<
> +				 PAGE_SHIFT;
> +	unsigned long rounded_page_addr = ALIGN_DOWN(fault_addr, pagesize);
> +	unsigned long rounded_page_end = rounded_page_addr + pagesize;
> +	unsigned long fault_offset;
> +	unsigned long fault_offset_end;
> +	unsigned long range_start_offset = 0;
> +	unsigned int i;
> +	int ret;
> +
> +	if (unlikely(!vdev))
> +		return -ENODEV;
> +
> +	/* This prevents the dmabuf revocation state from changing under us */
> +	lockdep_assert_held(&vdev->dmabuf_lock);
> +
> +	if (unlikely(priv->vdev != vdev || priv->revoked))
> +		return -ENODEV;

All but the lockdep here smells a bit like the hardening Leon mentions
in the previous patch.  There's only one caller of this, we'd segfault
there if vdev were NULL and the caller justifies how it knows vdev is
not NULL.  vdev is derived from priv->vdev, so the inequality can never
be true.  The lockdep annotation is good, but that also indicates the
caller is responsible for holding the revocation state stable, which
they've already checked.  I won't draw a hard line on these, but they
seem a bit pedantic.

> +
> +	if (rounded_page_addr < vma->vm_start || rounded_page_end > vma->vm_end) {
> +		if (order > 0)
> +			return -ERANGE;
> +
> +		/* A fault address outside of the VMA is absurd. */
> +		dev_warn_ratelimited(
> +			&vdev->pdev->dev,
> +			"Fault addr 0x%lx outside VMA 0x%lx-0x%lx\n",
> +			fault_addr, vma->vm_start, vma->vm_end);
> +		return -EFAULT;
> +	}
> +
> +	/*
> +	 * fault_offset[_end] is the span within the DMABUF
> +	 * corresponding to the faulting page:
> +	 */
> +	if (unlikely(check_add_overflow(rounded_page_addr - vma->vm_start,
> +					vma_off, &fault_offset) ||
> +		     check_add_overflow(fault_offset, pagesize,
> +					&fault_offset_end)))
> +		return -EFAULT;
> +
> +	/*
> +	 * Iterate over ranges in the buffer, summing their lengths:
> +	 * range_start_offset represents the current range's starting
> +	 * offset in the buffer (from 0 upwards).
> +	 *
> +	 * A failure for order == 0 is unexpected, and triggers a
> +	 * fault/warn.
> +	 */
> +	ret = (order == 0) ? -EFAULT : -ERANGE;
> +
> +	for (i = 0; i < priv->nr_ranges; i++) {
> +		size_t range_len = priv->phys_vec[i].len;
> +
> +		/* Early exit if range starts after the page end */
> +		if (fault_offset_end <= range_start_offset)
> +			break;
> +
> +		if (fault_offset >= range_start_offset &&
> +		    fault_offset_end <= range_start_offset + range_len) {
> +			/*
> +			 * The faulting page is wholly contained
> +			 * within the span represented by this range,
> +			 * so validate PFN alignment for the order.
> +			 * The if() condition ensures the pfn
> +			 * arithmetic won't overflow.
> +			 */
> +			unsigned long pfn =
> +				((fault_offset - range_start_offset) +
> +				 priv->phys_vec[i].paddr) >> PAGE_SHIFT;
> +
> +			if (IS_ALIGNED(pfn, 1 << order)) {
> +				*out_pfn = pfn;
> +				ret = 0;
> +			}
> +			/*
> +			 * Else order > 0; ERANGE retries with smaller
> +			 * order
> +			 */
> +			break;
> +		}
> +		range_start_offset += range_len;
> +	}
> +
> +	if (order == 0 && ret != 0)
> +		/*
> +		 * The address fell outside of the span represented by
> +		 * the (concatenated) ranges.  As setup of a mapping
> +		 * ensures that the VMA is <= the total size of the
> +		 * ranges this should never happen.  If it does, warn
> +		 * and SIGBUS.
> +		 */
> +		dev_warn_ratelimited(
> +			&vdev->pdev->dev,
> +			"No range for addr 0x%lx, order %d: VMA 0x%lx-0x%lx pgoff 0x%lx, %u ranges, size 0x%zx\n",
> +			fault_addr, order, vma->vm_start, vma->vm_end,
> +			vma->vm_pgoff, priv->nr_ranges, priv->size);
> +
> +	return ret;
> +}
> +
>  /*
>   * This is a temporary "private interconnect" between VFIO DMABUF and iommufd.
>   * It allows the two co-operating drivers to exchange the physical address of
> diff --git a/drivers/vfio/pci/vfio_pci_priv.h b/drivers/vfio/pci/vfio_pci_priv.h
> index 4e7162234a2e..786df3a81d9d 100644
> --- a/drivers/vfio/pci/vfio_pci_priv.h
> +++ b/drivers/vfio/pci/vfio_pci_priv.h
> @@ -23,6 +23,20 @@ struct vfio_pci_ioeventfd {
>  	bool			test_mem;
>  };
>  
> +struct vfio_pci_dma_buf {
> +	struct dma_buf *dmabuf;
> +	struct vfio_pci_core_device *vdev;
> +	struct list_head dmabufs_elm;
> +	size_t size;
> +	struct phys_vec *phys_vec;
> +	struct p2pdma_provider *provider;
> +	u32 nr_ranges;
> +	struct kref kref;
> +	struct completion comp;
> +	unsigned long vma_pgoff_adjust;
> +	u8 revoked : 1;
> +};
> +
>  bool vfio_pci_intx_mask(struct vfio_pci_core_device *vdev);
>  void vfio_pci_intx_unmask(struct vfio_pci_core_device *vdev);
>  
> @@ -123,6 +137,13 @@ static inline bool vfio_pci_is_vga(struct pci_dev *pdev)
>  	return (pdev->class >> 8) == PCI_CLASS_DISPLAY_VGA;
>  }
>  
> +int vfio_pci_dma_buf_find_pfn(struct vfio_pci_core_device *vdev,
> +			      struct vfio_pci_dma_buf *priv,
> +			      struct vm_area_struct *vma,
> +			      unsigned long address,

Nit, address in the prototype here is fault_addr in the actual function
definition.  Thanks,

Alex

> +			      unsigned int order,
> +			      unsigned long *out_pfn);
> +
>  #ifdef CONFIG_VFIO_PCI_DMABUF
>  int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
>  				  struct vfio_device_feature_dma_buf __user *arg,