On 1/8/26 12:04, Shameer Kolothum wrote:
-----Original Message----- From: Duan, Zhenzhong <[email protected]> Sent: 08 January 2026 09:41 To: Shameer Kolothum <[email protected]>; qemu- [email protected]; [email protected] Cc: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; Nicolin Chen <[email protected]>; Nathan Chen <[email protected]>; Matt Ochs <[email protected]>; Jason Gunthorpe <[email protected]>; Krishnakant Jaju <[email protected]> Subject: Re: [PATCH 3/3] hw/vfio/region: Create dmabuf for PCI BAR per region External email: Use caution opening links or attachments On 12/22/2025 9:53 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 pagetable.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 | 57+++++++++++++++++++++++++++++++++++++++++++-hw/vfio/trace-events | 1 + 2 files changed, 57 insertions(+), 1 deletion(-) diff --git a/hw/vfio/region.c b/hw/vfio/region.c index b165ab0b93..6949f6779c 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 +239,52 @@ 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); + + 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 = vbasedev->io_ops->device_feature(vbasedev, feature);vbasedev->io_ops->device_feature may be NULL for other backend like vfio- user.Ah..Ok. I will add a check.
Could you please add a global routine : int vfio_device_get_feature(VFIODevice *vbasedev, struct vfio_device_feature *feature)
+ for (i = 0; i < region->nr_mmaps; i++) { + trace_vfio_region_dmabuf(region->vbasedev->name, ret, region->nr, + region->mem->name, 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) {Just curious, when will above check return true?I think `!region->mem` covers cases where no MemoryRegion was created (e.g. zero sized regions). And nr_mmaps checks regions with mmap support exists (VFIO_REGION_INFO_FLAG_MMAP/ _CAP_SPARSE_MMAP).return 0; } @@ -305,6 +345,21 @@ 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 dmabuf not supported in kernel"); + } else { + error_report("%s: failed to create dmabuf: %s", + memory_region_name(region->mem), strerror(errno));
Shouldn't we return 'ret' in this case ? Thanks, C.
+ } + } else { + MemoryRegion *mr = ®ion->mmaps[0].mem;Do we need to support region->mmaps[1]?My understanding is all region->mmaps[] entries for a VFIO region share the same RAMBlock. And the kernel returns a single dmabuf fd per region, not per subrange. Thanks, ShameerThanks Zhenzhong+ RAMBlock *ram_block = mr->ram_block; + + ram_block->fd = ret; + } + 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_tdata, 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%"PRIx64vfio_region_setup(const char *dev, int index, const char *name, unsignedlong 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 longend) "Region %s [0x%lx - 0x%lx]"vfio_region_exit(const char *name, int index) "Device %s, region %d"
