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Wed, 27 May 2026 18:38:38 -0400 (EDT) Date: Wed, 27 May 2026 16:38:37 -0600 From: Alex Williamson To: Matt Evans Cc: Leon Romanovsky , Jason Gunthorpe , Alex Mastro , Christian =?UTF-8?B?S8O2bmln?= , Bjorn Helgaas , Logan Gunthorpe , Mahmoud Adam , David Matlack , =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= , Sumit Semwal , Kevin Tian , Ankit Agrawal , Pranjal Shrivastava , Alistair Popple , Vivek Kasireddy , , , , , , , alex@shazbot.org Subject: Re: [PATCH v2 2/9] vfio/pci: Add a helper to look up PFNs for DMABUFs Message-ID: <20260527163837.02be1e7c@shazbot.org> In-Reply-To: <20260527102319.100128-3-mattev@meta.com> References: <20260527102319.100128-1-mattev@meta.com> <20260527102319.100128-3-mattev@meta.com> X-Mailer: Claws Mail 4.4.0 (GTK 3.24.51; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" On Wed, 27 May 2026 03:23:05 -0700 Matt Evans wrote: > Add vfio_pci_dma_buf_find_pfn(), which a VMA fault handler can use to > find a PFN. > > This supports multi-range DMABUFs, which typically would be used to > represent scattered spans but might even represent overlapping or > aliasing spans of PFNs. > > Because this is intended to be used in vfio_pci_core.c, we also need > to expose the struct vfio_pci_dma_buf in the vfio_pci_priv.h header. > > Signed-off-by: Matt Evans > --- > drivers/vfio/pci/vfio_pci_dmabuf.c | 142 ++++++++++++++++++++++++++--- > drivers/vfio/pci/vfio_pci_priv.h | 20 ++++ > 2 files changed, 149 insertions(+), 13 deletions(-) > > diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c > index c16f460c01d6..0d132c4ca95f 100644 > --- a/drivers/vfio/pci/vfio_pci_dmabuf.c > +++ b/drivers/vfio/pci/vfio_pci_dmabuf.c > @@ -9,19 +9,6 @@ > > MODULE_IMPORT_NS("DMA_BUF"); > > -struct vfio_pci_dma_buf { > - struct dma_buf *dmabuf; > - struct vfio_pci_core_device *vdev; > - struct list_head dmabufs_elm; > - size_t size; > - struct phys_vec *phys_vec; > - struct p2pdma_provider *provider; > - u32 nr_ranges; > - struct kref kref; > - struct completion comp; > - u8 revoked : 1; > -}; > - > static int vfio_pci_dma_buf_attach(struct dma_buf *dmabuf, > struct dma_buf_attachment *attachment) > { > @@ -106,6 +93,135 @@ static const struct dma_buf_ops vfio_pci_dmabuf_ops = { > .release = vfio_pci_dma_buf_release, > }; > > +int vfio_pci_dma_buf_find_pfn(struct vfio_pci_dma_buf *vpdmabuf, This private dmabuf object pointer is named priv pretty consistently throughout the rest of the file. > + struct vm_area_struct *vma, > + unsigned long address, > + unsigned int order, > + unsigned long *out_pfn) > +{ > + /* > + * Given a VMA (start, end, pgoffs) and a fault address, > + * search the corresponding DMABUF's phys_vec[] to find the > + * range representing the address's offset into the VMA, and > + * its PFN. > + * > + * The phys_vec[] ranges represent contiguous spans of VAs > + * upwards from the buffer offset 0; the actual PFNs might be > + * in any order, overlap/alias, etc. Calculate an offset of > + * the desired page given VMA start/pgoff and address, then > + * search upwards from 0 to find which span contains it. > + * > + * On success, a valid PFN for a page sized by 'order' is > + * returned into out_pfn. > + * > + * Failure occurs if: > + * - The page would cross the edge of the VMA > + * - The page isn't entirely contained within a range > + * - We find a range, but the final PFN isn't aligned to the > + * requested order. > + * > + * (Upon failure, the caller is expected to try again with a > + * smaller order; the tests above will always succeed for > + * order=0 as the limit case.) > + * > + * It's suboptimal if DMABUFs are created with neigbouring > + * ranges that are physically contiguous, since hugepages > + * can't straddle range boundaries. (The construction of the > + * ranges vector should merge such ranges.) > + * > + * Finally, vma_pgoff_adjust is used for a DMABUF representing > + * a VFIO BAR mmap, which is created from the start of the > + * offset region. It should be zero, or equal vm_pgoff. > + */ > + > + const unsigned long pagesize = PAGE_SIZE << order; > + unsigned long vma_off = ((vma->vm_pgoff - vpdmabuf->vma_pgoff_adjust) << > + PAGE_SHIFT) & VFIO_PCI_OFFSET_MASK; > + unsigned long rounded_page_addr = ALIGN_DOWN(address, pagesize); > + unsigned long rounded_page_end = rounded_page_addr + pagesize; > + unsigned long page_buf_offset; > + unsigned long range_buf_offset = 0; > + unsigned int i; > + > + if (rounded_page_addr < vma->vm_start || rounded_page_end > vma->vm_end) { > + if (order > 0) > + return -EAGAIN; > + > + /* A fault address outside of the VMA is absurd. */ > + WARN(1, "Fault addr 0x%lx outside VMA 0x%lx-0x%lx\n", > + address, vma->vm_start, vma->vm_end); > + return -EFAULT; > + } > + > + if (vpdmabuf->vma_pgoff_adjust != 0 && > + vpdmabuf->vma_pgoff_adjust != (vma->vm_pgoff & > + (VFIO_PCI_OFFSET_MASK >> PAGE_SHIFT))) { > + WARN(1, "Unexpected vma_pgoff_adjust 0x%lx (vm_pgoff 0x%lx)\n", > + vpdmabuf->vma_pgoff_adjust, vma->vm_pgoff); > + return -EFAULT; > + } This appears to be user trigger'able, by causing a vma to be split, and also unnecessary as it seems the math works out correctly for the split case. > + > + if (unlikely(check_add_overflow(rounded_page_addr - vma->vm_start, > + vma_off, &page_buf_offset))) > + return -EFAULT; > + > + for (i = 0; i < vpdmabuf->nr_ranges; i++) { > + unsigned long page_buf_offset_end; > + size_t range_len = vpdmabuf->phys_vec[i].len; > + phys_addr_t range_start = vpdmabuf->phys_vec[i].paddr; > + > + if (unlikely(check_add_overflow(page_buf_offset, pagesize, > + &page_buf_offset_end))) > + return -EFAULT; Why is this inside the loop, the args are invariant of anything modified in the loop. Thanks, Alex > + /* > + * If the current range starts after the page's span, > + * this and any future range won't match. Bail early. > + */ > + if (page_buf_offset_end <= range_buf_offset) > + break; > + > + if (page_buf_offset >= range_buf_offset && > + page_buf_offset_end <= range_buf_offset + range_len) { > + /* > + * The faulting page is wholly contained > + * within the span represented by the range. > + * Validate PFN alignment for the order: > + */ > + unsigned long pfn = (range_start + page_buf_offset - > + range_buf_offset) / PAGE_SIZE; > + > + if (IS_ALIGNED(pfn, 1 << order)) { > + *out_pfn = pfn; > + return 0; > + } > + /* Retry with smaller order */ > + return -EAGAIN; > + } > + range_buf_offset += range_len; > + } > + > + /* > + * A hugepage straddling a range boundary will fail to match a > + * range, but the address will (eventually) match when retried > + * with a smaller page. > + */ > + if (order > 0) > + return -EAGAIN; > + > + /* > + * If we get here, the address fell outside of the span > + * represented by the (concatenated) ranges. Setup of a > + * mapping must ensure that the VMA is <= the total size of > + * the ranges, so this should never happen. But, if it does, > + * force SIGBUS for the access and warn. > + */ > + WARN_ONCE(1, "No range for addr 0x%lx, order %d: VMA 0x%lx-0x%lx pgoff 0x%lx, %u ranges, size 0x%zx\n", > + address, order, vma->vm_start, vma->vm_end, vma->vm_pgoff, > + vpdmabuf->nr_ranges, vpdmabuf->size); > + > + return -EFAULT; > +} > + > /* > * This is a temporary "private interconnect" between VFIO DMABUF and iommufd. > * It allows the two co-operating drivers to exchange the physical address of > diff --git a/drivers/vfio/pci/vfio_pci_priv.h b/drivers/vfio/pci/vfio_pci_priv.h > index fca9d0dfac90..c8f6f959056a 100644 > --- a/drivers/vfio/pci/vfio_pci_priv.h > +++ b/drivers/vfio/pci/vfio_pci_priv.h > @@ -23,6 +23,20 @@ struct vfio_pci_ioeventfd { > bool test_mem; > }; > > +struct vfio_pci_dma_buf { > + struct dma_buf *dmabuf; > + struct vfio_pci_core_device *vdev; > + struct list_head dmabufs_elm; > + size_t size; > + struct phys_vec *phys_vec; > + struct p2pdma_provider *provider; > + u32 nr_ranges; > + struct kref kref; > + struct completion comp; > + unsigned long vma_pgoff_adjust; > + u8 revoked : 1; > +}; > + > bool vfio_pci_intx_mask(struct vfio_pci_core_device *vdev); > void vfio_pci_intx_unmask(struct vfio_pci_core_device *vdev); > > @@ -114,6 +128,12 @@ static inline bool vfio_pci_is_vga(struct pci_dev *pdev) > return (pdev->class >> 8) == PCI_CLASS_DISPLAY_VGA; > } > > +int vfio_pci_dma_buf_find_pfn(struct vfio_pci_dma_buf *vpdmabuf, > + struct vm_area_struct *vma, > + unsigned long address, > + unsigned int order, > + unsigned long *out_pfn); > + > #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,