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

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Message-ID: <20250803155906.GM26511@ziepe.ca>
Date: Sun, 3 Aug 2025 12:59:06 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Matthew Wilcox <willy@...radead.org>
Cc: Robin Murphy <robin.murphy@....com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Christoph Hellwig <hch@....de>, Leon Romanovsky <leon@...nel.org>,
	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 Thu, Jul 31, 2025 at 06:37:11PM +0100, Matthew Wilcox wrote:

> The replacement for kmap_atomic() is already here -- it's
> kmap_(atomic|local)_pfn().  If a simple wrapper like kmap_local_phys()
> would make this more palatable, that would be fine by me.  Might save
> a bit of messing around with calculating offsets in each caller.

I think that makes the general plan clearer. We should be removing the
struct pages entirely from the insides of DMA API layer and use the
phys_addr_t, kmap_XX_phys(), phys_to_virt(), and so on.

The request from Christoph and Marek to clean up the dma_ops makes
sense in that context, we'd have to go into the ops and replace the
struct page kmaps/etc with the phys based ones.

This hides the struct page requirement to get to a KVA inside the core
mm code only and that sort of modularity is exactly the sort of thing
that could help entirely remove a struct page requirement for some
kinds of DMA someday.

Matthew, do you think it makes sense to introduce types to make this
clearer? We have two kinds of values that a phys_addr_t can store -
something compatible with kmap_XX_phys(), and something that isn't.

This was recently a long discussion in ARM KVM as well which had a
similar confusion that a phys_addr_t was actually two very different
things inside its logic.

So what about some dedicated types:
 kphys_addr_t - A physical address that can be passed to
     kmap_XX_phys(), phys_to_virt(), etc.

 raw_phys_addr_t - A physical address that may not be cachable, may
     not be DRAM, and does not work with kmap_XX_phys()/etc.

We clearly have these two different ideas floating around in code,
page tables, etc.

I read some of Robin's concern that the struct page provided a certain
amount of type safety in the DMA API, this could provide similar.

Thanks,
Jason