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

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Message-ID: <20251017115524.GG3901471@nvidia.com>
Date: Fri, 17 Oct 2025 08:55:24 -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 Thu, Oct 16, 2025 at 11:32:59PM -0700, Christoph Hellwig wrote:
> On Mon, Oct 13, 2025 at 06:26:10PM +0300, Leon Romanovsky wrote:
> > From: Leon Romanovsky <leonro@...dia.com>
> > 
> > Make sure that all VFIO PCI devices have peer-to-peer capabilities
> > enables, so we would be able to export their MMIO memory through DMABUF,
> 
> How do you know that they are safe to use with P2P?

All PCI devices are "safe" for P2P by spec. I've never heard of a
non-complaint device causing problems in this area.

The issue is always SOC support inside the CPU and that is delt with
inside the P2P subsystem logic.

If we ever see a problem it would be delt with by quirking the broken
device through pci-quirks and having the p2p subsystem refuse any p2p
with that device.

Jason