linux-kernel - Re: [PATCH 00/10] vfio/pci: Allow MMIO regions to be exported through dma-buf

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Message-ID: <20250730135846.2208fe89.alex.williamson@redhat.com>
Date: Wed, 30 Jul 2025 13:58:46 -0600
From: Alex Williamson <alex.williamson@...hat.com>
To: Leon Romanovsky <leon@...nel.org>
Cc: Leon Romanovsky <leonro@...dia.com>, Christoph Hellwig <hch@....de>,
 Jason Gunthorpe <jgg@...dia.com>, Andrew Morton
 <akpm@...ux-foundation.org>, Bjorn Helgaas <bhelgaas@...gle.com>, Christian
 König <christian.koenig@....com>,
 dri-devel@...ts.freedesktop.org, iommu@...ts.linux.dev, Jens Axboe
 <axboe@...nel.dk>, Jérôme Glisse <jglisse@...hat.com>,
 Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
 linaro-mm-sig@...ts.linaro.org, linux-block@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
 linux-mm@...ck.org, linux-pci@...r.kernel.org, Logan Gunthorpe
 <logang@...tatee.com>, Marek Szyprowski <m.szyprowski@...sung.com>, Robin
 Murphy <robin.murphy@....com>, Sumit Semwal <sumit.semwal@...aro.org>,
 Vivek Kasireddy <vivek.kasireddy@...el.com>, Will Deacon <will@...nel.org>
Subject: Re: [PATCH 00/10] vfio/pci: Allow MMIO regions to be exported
 through dma-buf

On Wed, 23 Jul 2025 16:00:01 +0300
Leon Romanovsky <leon@...nel.org> wrote:

> From: Leon Romanovsky <leonro@...dia.com>
> 
> ---------------------------------------------------------------------------
> Based on blk and DMA patches which will be sent during coming merge window.
> ---------------------------------------------------------------------------
> 
> This series extends the VFIO PCI subsystem to support exporting MMIO regions
> from PCI device BARs as dma-buf objects, enabling safe sharing of non-struct
> page memory with controlled lifetime management. This allows RDMA and other
> subsystems to import dma-buf FDs and build them into memory regions for PCI
> P2P operations.
> 
> The series supports a use case for SPDK where a NVMe device will be owned
> by SPDK through VFIO but interacting with a RDMA device. The RDMA device
> may directly access the NVMe CMB or directly manipulate the NVMe device's
> doorbell using PCI P2P.
> 
> However, as a general mechanism, it can support many other scenarios with
> VFIO. This dmabuf approach can be usable by iommufd as well for generic
> and safe P2P mappings.

I think this will eventually enable DMA mapping of device MMIO through
an IOMMUFD IOAS for the VM P2P use cases, right?  How do we get from
what appears to be a point-to-point mapping between two devices to a
shared IOVA between multiple devices?  I'm guessing we need IOMMUFD to
support something like IOMMU_IOAS_MAP_FILE for dma-buf, but I can't
connect all the dots.  Thanks,

Alex