On 1/13/2026 7:37 PM, Shameer Kolothum wrote:
From: Nicolin Chen <[email protected]>
Linux now provides a VFIO dmabuf exporter to expose PCI BAR memory for P2P
use cases. Create a dmabuf for each mapped BAR region after the mmap is set
up, and store the returned fd in the region’s RAMBlock. This allows QEMU to
pass the fd to dma_map_file(), enabling iommufd to import the dmabuf and map
the BAR correctly in the host IOMMU page table.
If the kernel lacks support or dmabuf setup fails, QEMU skips the setup
and continues with normal mmap handling.
Signed-off-by: Nicolin Chen <[email protected]>
Signed-off-by: Shameer Kolothum <[email protected]>
---
hw/vfio/region.c | 69 +++++++++++++++++++++++++++++++++++++++++++-
hw/vfio/trace-events | 1 +
2 files changed, 69 insertions(+), 1 deletion(-)
diff --git a/hw/vfio/region.c b/hw/vfio/region.c
index ca75ab1be4..8476a54dda 100644
--- a/hw/vfio/region.c
+++ b/hw/vfio/region.c
@@ -29,6 +29,7 @@
#include "qemu/error-report.h"
#include "qemu/units.h"
#include "monitor/monitor.h"
+#include "system/ramblock.h"
#include "vfio-helpers.h"
/*
@@ -238,13 +239,53 @@ static void vfio_subregion_unmap(VFIORegion *region, int
index)
region->mmaps[index].mmap = NULL;
}
+static int vfio_region_create_dma_buf(VFIORegion *region)
+{
+ g_autofree struct vfio_device_feature *feature = NULL;
+ VFIODevice *vbasedev = region->vbasedev;
+ struct vfio_device_feature_dma_buf *dma_buf;
+ size_t total_size;
+ int i, ret;
+
+ g_assert(region->nr_mmaps);
It's redundant as you have the check in vfio_region_mmap(), it's trivial.
+
+ total_size = sizeof(*feature) + sizeof(*dma_buf) +
+ sizeof(struct vfio_region_dma_range) * region->nr_mmaps;
+ feature = g_malloc0(total_size);
+ *feature = (struct vfio_device_feature) {
+ .argsz = total_size,
+ .flags = VFIO_DEVICE_FEATURE_GET | VFIO_DEVICE_FEATURE_DMA_BUF,
+ };
+
+ dma_buf = (void *)feature->data;
+ *dma_buf = (struct vfio_device_feature_dma_buf) {
+ .region_index = region->nr,
+ .open_flags = O_RDWR,
+ .nr_ranges = region->nr_mmaps,
+ };
+
+ for (i = 0; i < region->nr_mmaps; i++) {
+ dma_buf->dma_ranges[i].offset = region->mmaps[i].offset;
+ dma_buf->dma_ranges[i].length = region->mmaps[i].size;
+ }
+
+ ret = vfio_device_get_feature(vbasedev, feature);
+ for (i = 0; i < region->nr_mmaps; i++) {
+ trace_vfio_region_dmabuf(region->vbasedev->name, ret, region->nr,
+ memory_region_name(region->mem),
+ region->mmaps[i].offset,
+ region->mmaps[i].size);
+ }
+ return ret;
+}
+
int vfio_region_mmap(VFIORegion *region)
{
int i, ret, prot = 0;
char *name;
int fd;
- if (!region->mem) {
+ if (!region->mem || !region->nr_mmaps) {
return 0;
}
@@ -305,6 +346,32 @@ int vfio_region_mmap(VFIORegion *region)
region->mmaps[i].size - 1);
}
+ ret = vfio_region_create_dma_buf(region);
+ if (ret < 0) {
+ if (ret == -ENOTTY) {
+ warn_report_once("VFIO dma-buf not supported in kernel: "
+ "PCI BAR IOMMU mappings may fail");
+ } else {
+ error_report("%s: failed to create dma-buf (%s): "
+ "PCI BAR IOMMU mappings may fail",
+ memory_region_name(region->mem), strerror(errno));
+ }
+ /*
+ * Return success as VFIO region mmap succeeded and dmabuf is optional.
+ * P2P DMA or exposing device memory use cases will not be supported.
+ */
+ return 0;
+ }
+
+ /* Assign the dmabuf fd to associated RAMBlock */
+ for (i = 0; i < region->nr_mmaps; i++) {
+ MemoryRegion *mr = ®ion->mmaps[i].mem;
+ RAMBlock *ram_block = mr->ram_block;
+
+ ram_block->fd = ret;
+ ram_block->fd_offset = region->mmaps[i].offset;
+ }
+
If you have next respin, maybe cleaner to move above dmabuf related code
into
vfio_region_create_dma_buf().Anyway LGTM,
Reviewed-by: Zhenzhong Duan <[email protected]>
return 0;
no_mmap:
diff --git a/hw/vfio/trace-events b/hw/vfio/trace-events
index 1e895448cd..592a0349d4 100644
--- a/hw/vfio/trace-events
+++ b/hw/vfio/trace-events
@@ -117,6 +117,7 @@ vfio_device_put(int fd) "close vdev->fd=%d"
vfio_region_write(const char *name, int index, uint64_t addr, uint64_t data, unsigned size) "
(%s:region%d+0x%"PRIx64", 0x%"PRIx64 ", %d)"
vfio_region_read(char *name, int index, uint64_t addr, unsigned size, uint64_t data) "
(%s:region%d+0x%"PRIx64", %d) = 0x%"PRIx64
vfio_region_setup(const char *dev, int index, const char *name, unsigned long flags, unsigned
long offset, unsigned long size) "Device %s, region %d \"%s\", flags: 0x%lx, offset:
0x%lx, size: 0x%lx"
+vfio_region_dmabuf(const char *dev, int fd, int index, const char *name, unsigned long offset,
unsigned long size) "Device %s, dmabuf fd %d region %d \"%s\", offset: 0x%lx, size:
0x%lx"
vfio_region_mmap_fault(const char *name, int index, unsigned long offset, unsigned long
size, int fault) "Region %s mmaps[%d], [0x%lx - 0x%lx], fault: %d"
vfio_region_mmap(const char *name, unsigned long offset, unsigned long end)
"Region %s [0x%lx - 0x%lx]"
vfio_region_exit(const char *name, int index) "Device %s, region %d"
