Hi Jason,
On 24/04/2026 19:15, Jason Gunthorpe wrote:
On Thu, Apr 16, 2026 at 06:17:45AM -0700, Matt Evans 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.
Signed-off-by: Matt Evans <[email protected]>
---
drivers/vfio/pci/vfio_pci_dmabuf.c | 124 ++++++++++++++++++++++++++---
drivers/vfio/pci/vfio_pci_priv.h | 19 +++++
2 files changed, 130 insertions(+), 13 deletions(-)
diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c
b/drivers/vfio/pci/vfio_pci_dmabuf.c
index 04478b7415a0..8b6bae56bbf2 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 +93,117 @@ 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_dma_buf *vpdmabuf,
+ struct vm_area_struct *vma,
+ unsigned long address,
+ 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.
+ *
+ * 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:
+ * - The page would cross the edge of the VMA
+ * - The page isn't entirely contained within a range
+ * - We find a range, but the final PFN isn't aligned to the
+ * requested order.
+ *
+ * (Upon failure, the caller is expected to try again with a
+ * smaller order; the tests above will always succeed for
+ * order=0 as the limit case.)
+ *
+ * It's suboptimal if DMABUFs are created with neigbouring
+ * ranges that are physically contiguous, since hugepages
+ * can't straddle range boundaries. (The construction of the
+ * ranges vector should merge such ranges.)
+ */
+
+ const unsigned long pagesize = PAGE_SIZE << order;
+ unsigned long rounded_page_addr = address & ~(pagesize - 1);
ALIGN_DOWN(address, pagesize);
Oops, right, fixed.
+ unsigned long rounded_page_end = rounded_page_addr + pagesize;
+ unsigned long buf_page_offset;
+ unsigned long buf_offset = 0;
+ unsigned int i;
+
+ if (rounded_page_addr < vma->vm_start || rounded_page_end >
vma->vm_end) {
+ if (order > 0)
+ return -EAGAIN;
+
+ /* A fault address outside of the VMA is absurd. */
+ WARN(1, "Fault addr 0x%lx outside VMA 0x%lx-0x%lx\n",
+ address, vma->vm_start, vma->vm_end);
+ return -EFAULT;
+ }
+
+ if (unlikely(check_add_overflow(rounded_page_addr - vma->vm_start,
+ vma->vm_pgoff << PAGE_SHIFT,
&buf_page_offset)))
+ return -EFAULT;
+
+ for (i = 0; i < vpdmabuf->nr_ranges; i++) {
+ size_t range_len = vpdmabuf->phys_vec[i].len;
+ phys_addr_t range_start = vpdmabuf->phys_vec[i].paddr;
+
+ /*
+ * If the current range starts after the page's span,
+ * this and any future range won't match. Bail early.
+ */
+ if (buf_page_offset + pagesize <= buf_offset)
+ break;
No overflow check on this +? If we are worried order is so large that
the first needs a check then this would too..
In the earlier check it's not order being large but the vm_pgoff, but
yes an overflow check wouldn't hurt here. Added.
I've found (my) choice of variable names here awkward, and have renamed
them to make it a bit clearer as to what's the page being searched for
and what's the range, etc.
+
+ if (buf_page_offset >= buf_offset &&
+ buf_page_offset + pagesize <= buf_offset + range_len) {
+ /*
+ * The faulting page is wholly contained
+ * within the span represented by the range.
+ * Validate PFN alignment for the order:
+ */
+ unsigned long pfn = (range_start >> PAGE_SHIFT) +
+ ((buf_page_offset - buf_offset) >> PAGE_SHIFT);
(range_start + (buf_page_offset - buf_offset)) / PAGE_SIZE;
WFM, done.
Thank you,
Matt
