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[34.124.168.21]) by smtp.gmail.com with ESMTPSA id 5a478bee46e88-31504d2ec2asm29148088eec.21.2026.07.30.15.55.28 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 30 Jul 2026 15:55:32 -0700 (PDT) Date: Thu, 30 Jul 2026 22:55:24 +0000 From: Pranjal Shrivastava To: Matt Evans Cc: Alex Williamson , Leon Romanovsky , Jason Gunthorpe , Alex Mastro , Christian =?iso-8859-1?Q?K=F6nig?= , Bjorn Helgaas , Logan Gunthorpe , Kevin Tian , Longfang Liu , Mahmoud Adam , David Matlack , =?iso-8859-1?Q?Bj=F6rn_T=F6pel?= , Sumit Semwal , Ankit Agrawal , Alistair Popple , Vivek Kasireddy , linux-kernel@vger.kernel.org, linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org, kvm@vger.kernel.org, linux-pci@vger.kernel.org Subject: Re: [PATCH v5 3/9] vfio/pci: Add a helper to look up PFNs for DMABUFs Message-ID: References: <20260715174737.15287-1-matt@ozlabs.org> <20260715174737.15287-4-matt@ozlabs.org> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260715174737.15287-4-matt@ozlabs.org> On Wed, Jul 15, 2026 at 06:47:26PM +0100, 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 | 153 ++++++++++++++++++++++++++--- > drivers/vfio/pci/vfio_pci_priv.h | 20 ++++ > 2 files changed, 160 insertions(+), 13 deletions(-) > > diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c > index c16f460c01d6..7c047400dfd1 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 +93,146 @@ 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 *priv, > + struct vm_area_struct *vma, > + unsigned long fault_addr, > + 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: > + * - A hugepage would cross the edge of the VMA, > + * - A hugepage isn't entirely contained within a range > + * (including where it straddles the boundary between > + * ranges), > + * - We find a range, but the final PFN isn't aligned to the > + * requested order. > + * > + * Upon failure, -EAGAIN is returned and the caller is > + * expected to try again with a smaller order, which will > + * eventually succeed (order=0 will always work). > + * > + * It's suboptimal if DMABUFs are created with neighbouring > + * ranges that are physically contiguous, since hugepages > + * can't straddle range boundaries. (The construction of the > + * ranges should merge them in this case.) > + * > + * Finally, vma_pgoff_adjust is used with a DMABUF created for > + * a VFIO BAR mmap: a BAR mapped with vm_pgoff > 0 creates a > + * DMABUF such that byte 0 of the VMA corresponds to byte 0 of > + * the DMABUF and byte 'vm_pgoff << PAGE_SHIFT' into the BAR. > + * To avoid double-offsetting in this scenario, subtracting > + * vma_pgoff_adjust from this (non-zero) vm_pgoff generates > + * the effective offset. > + */ > + > + const unsigned long pagesize = PAGE_SIZE << order; > + unsigned long vma_off = ((vma->vm_pgoff - priv->vma_pgoff_adjust) << > + PAGE_SHIFT) & VFIO_PCI_OFFSET_MASK; Maybe I'm getting ahead of myself here.. but it seems like this restricts us to only mapping DMABUFs at offsets < 1TB due to the VFIO_PCI_OFFSET_MASK (since we have HBMs on PCI devices now, hitting 1TB may not be a very distant future). While I understand this mask is needed to drop the BAR encoding in the high bits. My worry is, if in the future a user were to export a massive contiguous DMABUF (e.g., >1TB of aggregated HBM) and tried to mmap deep into it (passing an offset >= 1TB), this bitwise AND would silently drop the high bits, leading to silent data corruption. I think we should explicitly reject such an mmap with -EINVAL like: +const unsigned long pagesize = PAGE_SIZE << order; +unsigned long vma_off = (vma->vm_pgoff - priv->vma_pgoff_adjust) << PAGE_SHIFT; +/* + * Prevent silent wrap-around if the user mmaps a DMABUF at an + * offset greater than the VFIO index mask allows. + */ +if (unlikely(vma_off > VFIO_PCI_OFFSET_MASK)) + return -EINVAL; +vma_off &= VFIO_PCI_OFFSET_MASK; > + unsigned long rounded_page_addr = ALIGN_DOWN(fault_addr, pagesize); > + unsigned long rounded_page_end = rounded_page_addr + pagesize; > + unsigned long fault_offset; > + unsigned long fault_offset_end; > + unsigned long range_start_offset = 0; > + unsigned int i; > + int ret; > + Thanks, Praan