linux-kernel - Re: [PATCH 0/8] dma-mapping: migrate to physical address-based API

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Message-ID: <20250731060117.GR402218@unreal>
Date: Thu, 31 Jul 2025 09:01:17 +0300
From: Leon Romanovsky <leon@...nel.org>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: Matthew Wilcox <willy@...radead.org>,
	David Hildenbrand <david@...hat.com>,
	Robin Murphy <robin.murphy@....com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Christoph Hellwig <hch@....de>, Jonathan Corbet <corbet@....net>,
	Madhavan Srinivasan <maddy@...ux.ibm.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Jason Wang <jasowang@...hat.com>,
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
	Eugenio Pérez <eperezma@...hat.com>,
	Alexander Potapenko <glider@...gle.com>,
	Marco Elver <elver@...gle.com>, Dmitry Vyukov <dvyukov@...gle.com>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Jérôme Glisse <jglisse@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, iommu@...ts.linux.dev,
	virtualization@...ts.linux.dev, kasan-dev@...glegroups.com,
	linux-trace-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 0/8] dma-mapping: migrate to physical address-based API

On Wed, Jul 30, 2025 at 11:28:18AM -0300, Jason Gunthorpe wrote:
> On Wed, Jul 30, 2025 at 04:40:26PM +0300, Leon Romanovsky wrote:

<...>

> > The most reasonable way to prevent DMA_ATTR_SKIP_CPU_SYNC leakage is to
> > introduce new DMA attribute (let's call it DMA_ATTR_MMIO for now) and
> > pass it to both dma_map_phys() and dma_iova_link(). This flag will
> > indicate that p2p type is PCI_P2PDMA_MAP_THRU_HOST_BRIDGE and call to
> > right callbacks which will set IOMMU_MMIO flag and skip CPU sync,
> 
> So the idea is if the memory is non-cachable, no-KVA you'd call
> dma_iova_link(phys_addr, DMA_ATTR_MMIO) and dma_map_phys(phys_addr,
> DMA_ATTR_MMIO) ?

Yes

> 
> And then internally the dma_ops and dma_iommu would use the existing
> map_page/map_resource variations based on the flag, thus ensuring that
> MMIO is never kmap'd or cache flushed?
> 
> dma_map_resource is really then just
> dma_map_phys(phys_addr, DMA_ATTR_MMIO)?
> 
> I like this, I think it well addresses the concerns.

Yes, I had this idea and implementation before. :(

> 
> Jason
>