[Linaro-mm-sig] Re: [PATCH v2 4/9] vfio/pci: Convert BAR mmap() to use a DMABUF

On Wed, 27 May 2026 03:23:07 -0700 Matt Evans mattev@meta.com wrote:

Convert the VFIO device fd fops->mmap to create a DMABUF representing the BAR mapping, and make the VMA fault handler look up PFNs from the corresponding DMABUF. This supports future code mmap()ing BAR DMABUFs, and iommufd work to support Type1 P2P.

First, vfio_pci_core_mmap() uses the new vfio_pci_core_mmap_prep_dmabuf() helper to export a DMABUF representing a single BAR range. Then, the vfio_pci_mmap_huge_fault() callback is updated to understand revoked buffers, and uses the new vfio_pci_dma_buf_find_pfn() helper to determine the PFN for a given fault address.

Now that the VFIO DMABUFs can be mmap()ed, vfio_pci_dma_buf_move() zaps PTEs (used on the revocation and cleanup paths).

CONFIG_VFIO_PCI_CORE now unconditionally depends on CONFIG_DMA_SHARED_BUFFER and CONFIG_PCI_P2PDMA_CORE. The CONFIG_VFIO_PCI_DMABUF feature conditionally includes support for VFIO_DEVICE_FEATURE_DMA_BUF, depending on the availability of CONFIG_PCI_P2PDMA.

Signed-off-by: Matt Evans mattev@meta.com

drivers/vfio/pci/Kconfig | 4 +- drivers/vfio/pci/Makefile | 3 +- drivers/vfio/pci/vfio_pci_core.c | 79 +++++++++++++++++++----------- drivers/vfio/pci/vfio_pci_dmabuf.c | 12 +++++ drivers/vfio/pci/vfio_pci_priv.h | 11 +---- 5 files changed, 68 insertions(+), 41 deletions(-)

diff --git a/drivers/vfio/pci/Kconfig b/drivers/vfio/pci/Kconfig index 296bf01e185e..9197343a7301 100644 --- a/drivers/vfio/pci/Kconfig +++ b/drivers/vfio/pci/Kconfig @@ -6,6 +6,8 @@ config VFIO_PCI_CORE tristate select VFIO_VIRQFD select IRQ_BYPASS_MANAGER

• select PCI_P2PDMA_CORE
• select DMA_SHARED_BUFFER

config VFIO_PCI_INTX def_bool y if !S390 @@ -56,7 +58,7 @@ config VFIO_PCI_ZDEV_KVM To enable s390x KVM vfio-pci extensions, say Y. config VFIO_PCI_DMABUF

• def_bool y if VFIO_PCI_CORE && PCI_P2PDMA && DMA_SHARED_BUFFER

• def_bool y if PCI_P2PDMA

This largely only breaks consistency, but should VFIO_PCI_CORE become a 'depends on' rather than dropped entirely?

source "drivers/vfio/pci/mlx5/Kconfig" diff --git a/drivers/vfio/pci/Makefile b/drivers/vfio/pci/Makefile index 6138f1bf241d..881452ea89be 100644 --- a/drivers/vfio/pci/Makefile +++ b/drivers/vfio/pci/Makefile @@ -1,8 +1,7 @@ # SPDX-License-Identifier: GPL-2.0-only -vfio-pci-core-y := vfio_pci_core.o vfio_pci_intrs.o vfio_pci_rdwr.o vfio_pci_config.o +vfio-pci-core-y := vfio_pci_core.o vfio_pci_intrs.o vfio_pci_rdwr.o vfio_pci_config.o vfio_pci_dmabuf.o vfio-pci-core-$(CONFIG_VFIO_PCI_ZDEV_KVM) += vfio_pci_zdev.o -vfio-pci-core-$(CONFIG_VFIO_PCI_DMABUF) += vfio_pci_dmabuf.o obj-$(CONFIG_VFIO_PCI_CORE) += vfio-pci-core.o vfio-pci-y := vfio_pci.o diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c index 041243a84d81..c5f934905ce0 100644 --- a/drivers/vfio/pci/vfio_pci_core.c +++ b/drivers/vfio/pci/vfio_pci_core.c @@ -1683,18 +1683,6 @@ void vfio_pci_memory_unlock_and_restore(struct vfio_pci_core_device *vdev, u16 c up_write(&vdev->memory_lock); } -static unsigned long vma_to_pfn(struct vm_area_struct *vma) -{

• struct vfio_pci_core_device *vdev = vma->vm_private_data;
• int index = vma->vm_pgoff >> (VFIO_PCI_OFFSET_SHIFT - PAGE_SHIFT);
• u64 pgoff;
• pgoff = vma->vm_pgoff &

• ((1U << (VFIO_PCI_OFFSET_SHIFT - PAGE_SHIFT)) - 1);
  

• return (pci_resource_start(vdev->pdev, index) >> PAGE_SHIFT) + pgoff;

-}

vm_fault_t vfio_pci_vmf_insert_pfn(struct vfio_pci_core_device *vdev, struct vm_fault *vmf, unsigned long pfn, @@ -1722,23 +1710,42 @@ static vm_fault_t vfio_pci_mmap_huge_fault(struct vm_fault *vmf, unsigned int order) { struct vm_area_struct *vma = vmf->vma;

• struct vfio_pci_core_device *vdev = vma->vm_private_data;
• unsigned long addr = vmf->address & ~((PAGE_SIZE << order) - 1);
• unsigned long pgoff = (addr - vma->vm_start) >> PAGE_SHIFT;
• unsigned long pfn = vma_to_pfn(vma) + pgoff;
• vm_fault_t ret = VM_FAULT_FALLBACK;
• if (is_aligned_for_order(vma, addr, pfn, order)) {

• scoped_guard(rwsem_read, &vdev->memory_lock)
  

• 	ret = vfio_pci_vmf_insert_pfn(vdev, vmf, pfn, order);
  

• }

• struct vfio_pci_dma_buf *priv = vma->vm_private_data;
• struct vfio_pci_core_device *vdev;
• unsigned long pfn = 0;
• vm_fault_t ret = VM_FAULT_SIGBUS;

• dev_dbg_ratelimited(&vdev->pdev->dev,

• 	   "%s(,order = %d) BAR %ld page offset 0x%lx: 0x%x\n",
  

• 	    __func__, order,
  

• 	    vma->vm_pgoff >>
  

• 		(VFIO_PCI_OFFSET_SHIFT - PAGE_SHIFT),
  

• 	    pgoff, (unsigned int)ret);
  

• /*

• * We can rely on the existence of both a DMABUF (priv) and
  

• * the VFIO device it was exported from (vdev).  This fault's
  

• * VMA was established using vfio_pci_core_mmap_prep_dmabuf()
  

• * which transfers ownership of the VFIO device fd to the
  

• * DMABUF, and so the VFIO device is held open because the
  

• * VMA's vm_file (DMABUF) is open.
  

• *
  

• * Since vfio_pci_dma_buf_cleanup() cannot have happened,
  

• * vdev must be valid; we can take memory_lock.
  

• */
  

• vdev = READ_ONCE(priv->vdev);

The above comment argues that vdev is stable, so why do we need to access it with READ_ONCE()?

• scoped_guard(rwsem_read, &vdev->memory_lock) {

• if (!priv->revoked) {
  

• 	int pres = vfio_pci_dma_buf_find_pfn(priv, vma,
  

• 					     vmf->address,
  

• 					     order, &pfn);
  

• 	if (pres == 0)
  

• 		ret = vfio_pci_vmf_insert_pfn(vdev, vmf,
  

• 					      pfn, order);
  

• 	else if (pres == -EAGAIN)
  

• 		ret = VM_FAULT_FALLBACK;
  

• }
  

• dev_dbg_ratelimited(&vdev->pdev->dev,
  

• 		    "%s(order = %d) PFN 0x%lx, VA 0x%lx, pgoff 0x%lx: 0x%x\n",
  

• 		    __func__, order, pfn, vmf->address,
  

• 		    vma->vm_pgoff, (unsigned int)ret);
  

Looks like this should still be outside the scope of the memory_lock. Thanks,

Alex

• }

return ret; } @@ -1763,6 +1770,7 @@ int vfio_pci_core_mmap(struct vfio_device *core_vdev, struct vm_area_struct *vma unsigned int index; u64 phys_len, req_len, pgoff, req_start; void __iomem *bar_io;

• int ret;

index = vma->vm_pgoff >> (VFIO_PCI_OFFSET_SHIFT - PAGE_SHIFT); @@ -1802,7 +1810,20 @@ int vfio_pci_core_mmap(struct vfio_device *core_vdev, struct vm_area_struct *vma if (IS_ERR(bar_io)) return PTR_ERR(bar_io);

• vma->vm_private_data = vdev;

• /*

• * Create a DMABUF with a single range corresponding to this
  

• * mapping, and wire it into vma->vm_private_data.  The VMA's
  

• * vm_file becomes that of the DMABUF, and the DMABUF takes
  

• * ownership of the VFIO device file (put upon DMABUF
  

• * release).  This maintains the behaviour of a live VMA
  

• * mapping holding the VFIO device file open.
  

• */
  

• ret = vfio_pci_core_mmap_prep_dmabuf(vdev, vma,

• 			     pci_resource_start(pdev, index),
  

• 			     req_len, index);
  

• if (ret)

• return ret;
  

• vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot); vma->vm_page_prot = pgprot_decrypted(vma->vm_page_prot);

diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c index 782408c08a5e..f7797f58d44b 100644 --- a/drivers/vfio/pci/vfio_pci_dmabuf.c +++ b/drivers/vfio/pci/vfio_pci_dmabuf.c @@ -9,6 +9,7 @@ MODULE_IMPORT_NS("DMA_BUF"); +#ifdef CONFIG_VFIO_PCI_DMABUF static int vfio_pci_dma_buf_attach(struct dma_buf *dmabuf, struct dma_buf_attachment *attachment) { @@ -25,6 +26,7 @@ static int vfio_pci_dma_buf_attach(struct dma_buf *dmabuf, return 0; } +#endif /* CONFIG_VFIO_PCI_DMABUF */ static void vfio_pci_dma_buf_done(struct kref *kref) { @@ -89,7 +91,9 @@ static void vfio_pci_dma_buf_release(struct dma_buf *dmabuf) } static const struct dma_buf_ops vfio_pci_dmabuf_ops = { +#ifdef CONFIG_VFIO_PCI_DMABUF .attach = vfio_pci_dma_buf_attach, +#endif .map_dma_buf = vfio_pci_dma_buf_map, .unmap_dma_buf = vfio_pci_dma_buf_unmap, .release = vfio_pci_dma_buf_release, @@ -263,6 +267,7 @@ static int vfio_pci_dmabuf_export(struct vfio_pci_core_device *vdev, return 0; } +#ifdef CONFIG_VFIO_PCI_DMABUF /*

• This is a temporary "private interconnect" between VFIO DMABUF and iommufd.
• It allows the two co-operating drivers to exchange the physical address of

@@ -461,6 +466,7 @@ int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags, kfree(dma_ranges); return ret; } +#endif /* CONFIG_VFIO_PCI_DMABUF */ int vfio_pci_core_mmap_prep_dmabuf(struct vfio_pci_core_device *vdev, struct vm_area_struct *vma, @@ -535,6 +541,10 @@ void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev, bool revoked) struct vfio_pci_dma_buf *tmp; lockdep_assert_held_write(&vdev->memory_lock);

• /*

• * Holding memory_lock ensures a racing VMA fault observes
  

• * priv->revoked properly.
  

• */
  

list_for_each_entry_safe(priv, tmp, &vdev->dmabufs, dmabufs_elm) { if (!get_file_active(&priv->dmabuf->file)) @@ -552,6 +562,8 @@ void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev, bool revoked) if (revoked) { kref_put(&priv->kref, vfio_pci_dma_buf_done); wait_for_completion(&priv->comp);

• 		unmap_mapping_range(priv->dmabuf->file->f_mapping,
  

• 				    0, priv->size, 1);
  	/*
  	 * Re-arm the registered kref reference and the
  	 * completion so the post-revoke state matches the
  

diff --git a/drivers/vfio/pci/vfio_pci_priv.h b/drivers/vfio/pci/vfio_pci_priv.h index 06dc0fd3e230..d38e1b98b2e9 100644 --- a/drivers/vfio/pci/vfio_pci_priv.h +++ b/drivers/vfio/pci/vfio_pci_priv.h @@ -138,13 +138,13 @@ int vfio_pci_core_mmap_prep_dmabuf(struct vfio_pci_core_device *vdev, struct vm_area_struct *vma, u64 phys_start, u64 req_len, unsigned int res_index); +void vfio_pci_dma_buf_cleanup(struct vfio_pci_core_device *vdev); +void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev, bool revoked); #ifdef CONFIG_VFIO_PCI_DMABUF int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags, struct vfio_device_feature_dma_buf __user *arg, size_t argsz); -void vfio_pci_dma_buf_cleanup(struct vfio_pci_core_device *vdev); -void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev, bool revoked); #else static inline int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags, @@ -153,13 +153,6 @@ vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags, { return -ENOTTY; } -static inline void vfio_pci_dma_buf_cleanup(struct vfio_pci_core_device *vdev) -{ -} -static inline void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev,

• 			 bool revoked)
  

-{ -} #endif #endif