Re: [PATCH v13 7/9] vfio/device: Add a helper to mmap a dmabuf

[Cédric Le Goater]

On 10/7/26 22:09, Vivek Kasireddy wrote:

In order to mmap a dmabuf, we first need to create a placeholder
mapping that would fit the entire size of the dmabuf. This mapping
would then be replaced with smaller mappings of individual dmabuf
segments.

Cc: Alex Williamson <a...@shazbot.org>
Cc: Cédric Le Goater <c...@redhat.com>
Signed-off-by: Vivek Kasireddy <vivek.kasire...@intel.com>
---
  include/hw/vfio/vfio-device.h | 15 ++++++++++
  hw/vfio/device.c              | 55 +++++++++++++++++++++++++++++++++++
  hw/vfio/dmabuf-stubs.c        |  7 +++++
  3 files changed, 77 insertions(+)

diff --git a/include/hw/vfio/vfio-device.h b/include/hw/vfio/vfio-device.h
index c05430be8a..4b8fb62056 100644
--- a/include/hw/vfio/vfio-device.h
+++ b/include/hw/vfio/vfio-device.h
@@ -345,6 +345,21 @@ int vfio_device_get_irq_info(VFIODevice *vbasedev, int 
index,
   */
  int vfio_device_create_dmabuf_fd(struct iovec *iov, unsigned int iov_cnt,
                                   Error **errp);
+
+/**
+ * Mmap a dmabuf by first translating the addresses in the iovec
+ * array into VFIO region offsets and then creating a placeholder
+ * mapping that would be replaced later with mappings that
+ * correspond to the dmabuf segments.
+ *
+ * @iov: array of iovec entries associated with the dmabuf
+ * @iov_cnt: number of entries in the iovec array
+ * @errp: pointer to Error*, to store an error if it happens.
+ *
+ * Returns the mapped pointer on success and NULL on error.
+ */
+void *vfio_device_mmap_dmabuf(struct iovec *iov, unsigned int iov_cnt,
+                              Error **errp);
  #endif

/* Returns 0 on success, or a negative errno. */

diff --git a/hw/vfio/device.c b/hw/vfio/device.c
index d95503a5f1..5bc0443764 100644
--- a/hw/vfio/device.c
+++ b/hw/vfio/device.c
@@ -776,3 +776,58 @@ int vfio_device_create_dmabuf_fd(struct iovec *iov, 
unsigned int iov_cnt,
      }
      return ret;
  }
+
+void *vfio_device_mmap_dmabuf(struct iovec *iov, unsigned int iov_cnt,
+                              Error **errp)
+{
+    size_t total_size = iov_size(iov, iov_cnt);
+    struct vfio_region_info *info = NULL;
+    ram_addr_t offset, len = 0;
+    VFIODevice *vbasedev;
+    VFIORegion *region;
+    RAMBlock *first_rb;
+    void *map, *submap;
+    int i;
+
+    if (!vfio_device_lookup(iov, &vbasedev, &first_rb, errp)) {
+        return NULL;
+    }
+
+    /*
+     * We first reserve a contiguous chunk of address space for the entire
+     * dmabuf, then replace it with smaller mappings that correspond to the
+     * individual segments of the dmabuf.
+     */
+    map = mmap(NULL, total_size, PROT_NONE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
+    if (map == MAP_FAILED) {
+        error_setg_errno(errp, errno, "Could not reserve placeholder mapping");
+        return NULL;
+    }
+
+    for (i = 0; i < iov_cnt; i++) {
+        if (!vfio_region_lookup(iov[i].iov_base, &region,
+                                first_rb, &offset, errp)) {
+            goto err;
+        }
+
+        if (vfio_device_get_region_info(vbasedev, region->nr, &info)) {
+            error_setg(errp, "Cannot find region info for dmabuf segment\n");

no \n

+            goto err;
+        }
+
+        submap = mmap(map + len, iov[i].iov_len, PROT_READ,
+                      MAP_SHARED | MAP_FIXED, vbasedev->fd,
+                      info->offset + offset);
+        if (submap == MAP_FAILED) {
+            error_setg_errno(errp, errno, "Could not mmap dmabuf segment\n");

again.

+            goto err;
+        }
+        len += iov[i].iov_len;
+    }
+    return map;
+err:
+    munmap(map, total_size);
+    error_prepend(errp, "VFIO dmabuf mmap failed: ");
+    return NULL;
+}
+
diff --git a/hw/vfio/dmabuf-stubs.c b/hw/vfio/dmabuf-stubs.c
index fb5318e929..b9e03a162a 100644
--- a/hw/vfio/dmabuf-stubs.c
+++ b/hw/vfio/dmabuf-stubs.c
@@ -15,3 +15,10 @@ int vfio_device_create_dmabuf_fd(struct iovec *iov, unsigned 
int iov_cnt,
      error_setg(errp, "VFIO dmabuf support is not enabled");
      return VFIO_DMABUF_CREATE_ERR_UNSPEC;
  }
+
+void *vfio_device_mmap_dmabuf(struct iovec *iov, unsigned int iov_cnt,
+                              Error **errp)
+{
+    error_setg(errp, "VFIO mmap dmabuf support is not enabled");
+    return NULL;
+}