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

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Message-ID: <20250731002138.GD89283@nvidia.com>
Date: Wed, 30 Jul 2025 21:21:38 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Alex Williamson <alex.williamson@...hat.com>
Cc: Leon Romanovsky <leon@...nel.org>, Leon Romanovsky <leonro@...dia.com>,
	Christoph Hellwig <hch@....de>,
	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, Jul 30, 2025 at 01:58:46PM -0600, Alex Williamson wrote:
> 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?  

This is the plan

> How do we get from
> what appears to be a point-to-point mapping between two devices to a
> shared IOVA between multiple devices?

You have it right below, it is a point to point mapping between the
vfio device and the iommufd.

> I'm guessing we need IOMMUFD to support something like
> IOMMU_IOAS_MAP_FILE for dma-buf, 

1) The dma phys series which needs more work
2) This series to get basic 'movable' DMABUF support in VFIO
3) Add 'revokable' as a DMABUF concept and implement it with mlx5 and
   vfio
4) Add some way to get the phys_addr list from the DMABUF
5) IOMMU_IOAS_MAP_FILE using a revokable attachment and the phys_addr
   list. When VFIO does FLR the iommufd can remove the IOPTEs and then
   put them back when FLR is done.

It is not so much more code, but I think every step will take a lot of
work to get agreements.

Then we reuse all of the above with some tweaks for the CC problems
too.

Jason