Hi all,
This series is based on previous RFCs/discussions:
Tech topic: https://lore.kernel.org/linux-iommu/20250918214425.2677057-1-amastro@fb.com/ RFCv1: https://lore.kernel.org/all/20260226202211.929005-1-mattev@meta.com/ RFCv2: https://lore.kernel.org/kvm/20260312184613.3710705-1-mattev@meta.com/
The background/rationale is covered in more detail in the RFC cover letters. The TL;DR is:
The goal is to enable userspace driver designs that use VFIO to export DMABUFs representing subsets of PCI device BARs, and "vend" those buffers from a primary process to other subordinate processes by fd. These processes then mmap() the buffers and their access to the device is isolated to the exported ranges. This is an improvement on sharing the VFIO device fd to subordinate processes, which would allow unfettered access .
This is achieved by enabling mmap() of vfio-pci DMABUFs. Second, a new ioctl()-based revocation mechanism is added to allow the primary process to forcibly revoke access to previously-shared BAR spans, even if the subordinate processes haven't cleanly exited.
(The related topic of safe delegation of iommufd control to the subordinate processes is not addressed here, and is follow-up work.)
As well as isolation and revocation, another advantage to accessing a BAR through a VMA backed by a DMABUF is that it's straightforward to create the buffer with access attributes, such as write-combining.
Notes on patches ================
Feedback from the RFCs requested that, instead of creating DMABUF-specific vm_ops and .fault paths, to go the whole way and migrate the existing VFIO PCI BAR mmap() to be backed by a DMABUF too, resulting in a common vm_ops and fault handler for mmap()s of both the VFIO device and explicitly-exported DMABUFs. This has been done for vfio-pci, but not sub-drivers (nvgrace-gpu's special-case mappings are unchanged).
vfio/pci: Fix vfio_pci_dma_buf_cleanup() double-put
A bug fix to a related are, whose context is a depdency for later patches.
vfio/pci: Add a helper to look up PFNs for DMABUFs vfio/pci: Add a helper to create a DMABUF for a BAR-map VMA
The first is for a DMABUF VMA fault handler to determine arbitrary-sized PFNs from ranges in DMABUF. Secondly, refactor DMABUF export for use by the existing export feature and a new helper that creates a DMABUF corresponding to a VFIO BAR mmap() request.
vfio/pci: Convert BAR mmap() to use a DMABUF
The vfio-pci core mmap() creates a DMABUF with the helper, and the vm_ops fault handler uses the other helper to resolve the fault. Because this depends on DMABUF structs/code, CONFIG_VFIO_PCI_CORE needs to depend on CONFIG_DMA_SHARED_BUFFER. The CONFIG_VFIO_PCI_DMABUF still conditionally enables the export support code.
NOTE: The user mmap()s a device fd, but the resulting VMA's vm_file becomes that of the DMABUF which takes ownership of the device and puts it on release. This maintains the existing behaviour of a VMA keeping the VFIO device open.
BAR zapping then happens via the existing vfio_pci_dma_buf_move() path, which now needs to unmap PTEs in the DMABUF's address_space.
vfio/pci: Provide a user-facing name for BAR mappings
There was a request for decent debug naming in /proc/<pid>/maps etc. comparable to the existing VFIO names: since the VMAs are DMABUFs, they have a "dmabuf:" prefix and can't be 100% identical to before. This is a user-visible change, but this patch at least now gives us extra info on the BDF & BAR being mapped.
vfio/pci: Clean up BAR zap and revocation
In general (see NOTE!) the vfio_pci_zap_bars() is now obsolete, since it unmaps PTEs in the VFIO device address_space which is now unused. This consolidates all calls (e.g. around reset) with the neighbouring vfio_pci_dma_buf_move()s into new functions, to revoke-zap/unrevoke.
NOTE: the nvgrace-gpu driver continues to use its own private vm_ops, fault handler, etc. for its special memregions, and these DO still add PTEs to the VFIO device address_space. So, a temporary flag, vdev->bar_needs_zap, maintains the old behaviour for this use. At least this patch's consolidation makes it easy to remove the remaining zap when this need goes away.
A FIXME is added: if nvgrace-gpu is converted to DMABUFs, remove the flag and final zap.
vfio/pci: Support mmap() of a VFIO DMABUF
Adds mmap() for a DMABUF fd exported from vfio-pci.
It was a goal to keep the VFIO device fd lifetime behaviour unchanged with respect to the DMABUFs. An application can close all device fds, and this will revoke/clean up all DMABUFs; no mappings or other access can be performed now. When enabling mmap() of the DMABUFs, this means access through the VMA is also revoked. This complicates the fault handler because whilst the DMABUF exists, it has no guarantee that the corresponding VFIO device is still alive. Adds synchronisation ensuring the vdev is available before vdev->memory_lock is touched.
(I decided against the alternative of preventing cleanup by holding the VFIO device open if any DMABUFs exist, because it's both a change of behaviour and less clean overall.)
I've added a chonky comment in place, happy to clarify more if you have ideas.
vfio/pci: Permanently revoke a DMABUF on request
By weight, this is mostly a rename of revoked to an enum, status. There are now 3 states for a buffer, usable and revoked temporary/permanent. A new VFIO device ioctl is added, VFIO_DEVICE_PCI_DMABUF_REVOKE, which passes a DMABUF (exported from that device) and permanently revokes it. Thus a userspace driver can guarantee any downstream consumers of a shared fd are prevented from accessing a BAR range, and that range can be reused.
The code doing revocation in vfio_pci_dma_buf_move() is moved, unchanged, to a common function for use by _move() and the new ioctl path.
Q: I can't think of a good reason to temporarily revoke/unrevoke buffers from userspace, so didn't add a 'flags' field to the ioctl struct. Easy to add if people think it's worthwhile for future use.
vfio/pci: Add mmap() attributes to DMABUF feature
Reserves bits [31:28] in vfio_device_feature_dma_buf to allow a (CPU) mapping attribute to be specified for an exported set of ranges. The default is the current UC, and a new flag can specify CPU access as WC.
Q: I've taken 4 bits; the intention is for this field to be a scalar not a bitmap (i.e. mutually-exclusive access properties). Perhaps 4 is a bit too many?
Testing =======
(The [RFC ONLY] userspace test program, for QEMU edu-plus, has been dropped, but can be found in the GitHub branch below.)
This code has been tested in mapping DMABUFs of single/multiple ranges, aliasing mmap()s, aliasing ranges across DMABUFs, vm_pgoff > 0, revocation, shutdown/cleanup scenarios, and hugepage mappings seem to work correctly. I've lightly tested WC mappings also (by observing resulting PTEs as having the correct attributes...). No regressions observed on the VFIO selftests, or on our internal vfio-pci applications.
End ===
This is based on -next (next-20260414 but will merge earlier), as it depends on Leon's series "vfio: Wait for dma-buf invalidation to complete":
https://lore.kernel.org/linux-iommu/20260205-nocturnal-poetic-chamois-f566ad...
These commits are on GitHub, along with "[RFC ONLY] selftests: vfio: Add standalone vfio_dmabuf_mmap_test":
https://github.com/metamev/linux/compare/next-20260414...metamev:linux:dev/m...
Thanks for reading,
Matt
================================================================================ Change log:
v1:
- Cleanup of the common DMABUF-aware VMA vm_ops fault handler and export code. - Fixed a lot of races, particularly faults racing with DMABUF cleanup (if the VFIO device fds close, for example). - Added nicer human-readable names for VFIO mmap() VMAs
RFCv2: Respin based on the feedback/suggestions: https://lore.kernel.org/kvm/20260312184613.3710705-1-mattev@meta.com/
- Transform the existing VFIO BAR mmap path to also use DMABUFs behind the scenes, and then simply share that code for explicitly-mapped DMABUFs. Jason wanted to go that direction to enable iommufd VFIO type 1 emulation to pick up a DMABUF for an IO mapping.
- Revoke buffers using a VFIO device fd ioctl
RFCv1: https://lore.kernel.org/all/20260226202211.929005-1-mattev@meta.com/
Matt Evans (9): vfio/pci: Fix vfio_pci_dma_buf_cleanup() double-put vfio/pci: Add a helper to look up PFNs for DMABUFs vfio/pci: Add a helper to create a DMABUF for a BAR-map VMA vfio/pci: Convert BAR mmap() to use a DMABUF vfio/pci: Provide a user-facing name for BAR mappings vfio/pci: Clean up BAR zap and revocation vfio/pci: Support mmap() of a VFIO DMABUF vfio/pci: Permanently revoke a DMABUF on request vfio/pci: Add mmap() attributes to DMABUF feature
drivers/vfio/pci/Kconfig | 3 +- drivers/vfio/pci/Makefile | 3 +- drivers/vfio/pci/nvgrace-gpu/main.c | 5 + drivers/vfio/pci/vfio_pci_config.c | 30 +- drivers/vfio/pci/vfio_pci_core.c | 224 ++++++++++--- drivers/vfio/pci/vfio_pci_dmabuf.c | 500 +++++++++++++++++++++++----- drivers/vfio/pci/vfio_pci_priv.h | 49 ++- include/linux/vfio_pci_core.h | 1 + include/uapi/linux/vfio.h | 42 ++- 9 files changed, 690 insertions(+), 167 deletions(-)