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Date: Tue, 29 Jul 2025 12:41:00 +0200
From: Christoph Hellwig <hch@lst.de>
To: Leon Romanovsky <leon@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>, Jason Gunthorpe <jgg@nvidia.com>,
	Alex Williamson <alex.williamson@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Christian =?iso-8859-1?Q?K=F6nig?= <christian.koenig@amd.com>,
	dri-devel@lists.freedesktop.org, iommu@lists.linux.dev,
	Jens Axboe <axboe@kernel.dk>,
	=?iso-8859-1?B?Suly9G1l?= Glisse <jglisse@redhat.com>,
	Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
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	linux-mm@kvack.org, linux-pci@vger.kernel.org,
	Logan Gunthorpe <logang@deltatee.com>,
	Marek Szyprowski <m.szyprowski@samsung.com>,
	Robin Murphy <robin.murphy@arm.com>,
	Sumit Semwal <sumit.semwal@linaro.org>,
	Vivek Kasireddy <vivek.kasireddy@intel.com>,
	Will Deacon <will@kernel.org>
Subject: Re: [PATCH 02/10] PCI/P2PDMA: Introduce p2pdma_provider structure
 for cleaner abstraction
Message-ID: <20250729104100.GA29053@lst.de>
References: <cover.1753274085.git.leonro@nvidia.com> <c2307cb4c3f1af46da138f3410738754691fbb3d.1753274085.git.leonro@nvidia.com> <20250724075145.GB30590@lst.de> <20250724075533.GR402218@unreal> <20250724075922.GD30590@lst.de> <20250727185158.GE7551@nvidia.com> <20250729075209.GA23823@lst.de> <20250729085336.GG402218@unreal>
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User-Agent: Mutt/1.5.17 (2007-11-01)

On Tue, Jul 29, 2025 at 11:53:36AM +0300, Leon Romanovsky wrote:
> > Because the struct page is the only thing that:
> > 
> >  a) dma-mapping works on
> >  b) is the only place we can discover the routing information, but also
> >     more importantly ensure that the underlying page is still present
> >     and the device is not hot unplugged, or in a very theoretical worst
> >     case replaced by something else.
> 
> It is correct in general case, but here we are talking about MMIO
> memory, which is "connected" to device X and routing information is
> stable.

MMIO is literally the only thing we support to P2P to/from as that is
how PCIe P2P is defined.  And not, it's not stable - devices can be
unplugged, and BARs can be reenumerated.