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add > support for mmap() of this fd. This provides another route for a > process to map BARs, except one where the process can only map a specific > subset of a BAR represented by the exported DMABUF. > > mmap() support enables userspace driver designs that safely delegate > access to BAR sub-ranges to other client processes by sharing a DMABUF > fd, without having to share the (omnipotent) VFIO device fd with them. > > The mmap callback installs vm_ops callbacks for .fault and .huge_fault; > they find a PFN by searching the DMABUF's physical ranges. That is, > DMABUFs with multiple ranges are supported for mmap(). In general sounds like a good idea but this approach here doesn't looks good at all. Especially how you call unmap_mapping_range() from your DMA-buf cleanup path looks extremely questionable. ... > +/* > + * Similar to vfio_pci_core_mmap() for a regular VFIO device fd, but > + * differs by pre-checks performed and ultimately the vm_ops installed. > + */ > +static int vfio_pci_dma_buf_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma) > +{ > + struct vfio_pci_dma_buf *priv = dmabuf->priv; > + u64 req_len, req_start; > + > + if (!vfio_pci_dma_buf_is_mappable(dmabuf)) > + return -ENODEV; > + if ((vma->vm_flags & VM_SHARED) == 0) > + return -EINVAL; > + > + req_len = vma->vm_end - vma->vm_start; > + req_start = vma->vm_pgoff << PAGE_SHIFT; > + > + if (req_start + req_len > priv->size) > + return -EINVAL; > + > + vma->vm_private_data = priv; > + vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot); > + vma->vm_page_prot = pgprot_decrypted(vma->vm_page_prot); > + > + /* > + * See comments in vfio_pci_core_mmap() re VM_ALLOW_ANY_UNCACHED. > + * > + * FIXME: get mapping attributes from dmabuf? > + */ > + vm_flags_set(vma, VM_ALLOW_ANY_UNCACHED | VM_IO | VM_PFNMAP | > + VM_DONTEXPAND | VM_DONTDUMP); > + vma->vm_ops = &vfio_pci_dma_buf_mmap_ops; > + > + return 0; Let's start with this here, it just looks horrible over complicated. When a DMA-buf just represents a linear piece of BAR which is map-able through the VFIO FD anyway then the right approach is to just re-direct the mapping to this VFIO FD. Roughly something like this here should do it: vma->vm_pgoff += offset_which_your_dma_buf_represents; vma_set_file(vma, core_dev->file); vfio_pci_core_mmap(core_dev, vma); It can be that you want additional checks (e.g. if the DMA-buf is revoked) in which case you would need to override the vma->vm_ops, but then just do the access checks and call the vfio_pci_mmap_ops to get the actually page fault handling done. >+ unmap_mapping_range(priv->dmabuf->file->f_mapping, >+ 0, priv->size, 1); When you need to use unmap_mapping_range() then you usually share the address space object between the file descriptor exporting the DMA-buf and the DMA-buf fd itself. Otherwise functions like vfio_pci_zap_bars() doesn't work correctly any more and that usually creates a huge bunch of problems. Regards, Christian.