linux-kernel - Re: [PATCH v5 9/9] vfio/pci: Add dma-buf export support for MMIO regions

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Message-ID: <aPHjUP7pdIXuEPIq@infradead.org>
Date: Thu, 16 Oct 2025 23:33:52 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Leon Romanovsky <leon@...nel.org>
Cc: Alex Williamson <alex.williamson@...hat.com>,
	Leon Romanovsky <leonro@...dia.com>,
	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>, 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 v5 9/9] vfio/pci: Add dma-buf export support for MMIO
 regions

On Mon, Oct 13, 2025 at 06:26:11PM +0300, Leon Romanovsky wrote:
> From: Leon Romanovsky <leonro@...dia.com>
> 
> Add support for exporting PCI device MMIO regions through dma-buf,
> 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 implementation provides a revocable attachment mechanism using
> dma-buf move operations. MMIO regions are normally pinned as BARs
> don't change physical addresses, but access is revoked when the VFIO
> device is closed or a PCI reset is issued. This ensures kernel
> self-defense against potentially hostile userspace.

This still completely fails to explain why you think that it actually
is safe without the proper pgmap handling.