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

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Message-ID: <20251017121610.GI3901471@nvidia.com>
Date: Fri, 17 Oct 2025 09:16:10 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Christoph Hellwig <hch@...radead.org>
Cc: Leon Romanovsky <leon@...nel.org>,
	Alex Williamson <alex.williamson@...hat.com>,
	Leon Romanovsky <leonro@...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 Thu, Oct 16, 2025 at 11:33:52PM -0700, Christoph Hellwig wrote:
> 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.

Leon, this keeps coming up, please clean up and copy the text from my
prior email into the commit message & cover letter explaining in
detail how lifetime magement works by using revocation/invalidation
driven by dmabuf move_notify callbacks.

Jason