linux-kernel - Re: [PATCH v5 8/9] vfio/pci: Enable peer-to-peer DMA transactions by default

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Message-ID: <20251022113859.GC21107@nvidia.com>
Date: Wed, 22 Oct 2025 08:38:59 -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 8/9] vfio/pci: Enable peer-to-peer DMA transactions by
 default

On Wed, Oct 22, 2025 at 12:08:48AM -0700, Christoph Hellwig wrote:
> On Mon, Oct 20, 2025 at 10:08:55AM -0300, Jason Gunthorpe wrote:
> > Sure, but this should be handled by the P2P subsystem and PCI quirks,
> > IMHO. It isn't VFIOs job.. If people complain about broken HW then it
> > is easy to add those things.
> 
> I think it is.  You now open up behavior generally that previously
> had specific drivers in charge.

It has always been available in VFIO. This series is fixing it up to
not have the lifetime bugs.

> > IDK where Intel GPU lands on this, but VFIO has always supported P2P
> 
> How?

It uses follow_pfnmap_start()/etc to fish the MMIO PFNs out of a VMA and
stick them into the iommu.

Jason