Re: [PATCH v4 0/3] Apple M1 DART IOMMU driver

From: Robin Murphy <robin.murphy@arm.com>
To: Sven Peter <sven@svenpeter.dev>, Will Deacon <will@kernel.org>,
	Joerg Roedel <joro@8bytes.org>
Cc: Arnd Bergmann <arnd@kernel.org>,
	r.czerwinski@pengutronix.de, devicetree@vger.kernel.org,
	Marc Zyngier <maz@kernel.org>, Hector Martin <marcan@marcan.st>,
	linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org,
	Rob Herring <robh+dt@kernel.org>,
	Alexander Graf <graf@amazon.com>,
	Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>,
	Mohamed Mediouni <mohamed.mediouni@caramail.com>,
	Mark Kettenis <mark.kettenis@xs4all.nl>,
	linux-arm-kernel@lists.infradead.org,
	Stan Skowronek <stan@corellium.com>
Subject: Re: [PATCH v4 0/3] Apple M1 DART IOMMU driver
Date: Wed, 14 Jul 2021 19:19:50 +0100	[thread overview]
Message-ID: <7261df01-34a9-4e53-37cd-ae1aa15b1fb4@arm.com> (raw)
In-Reply-To: <20210627143405.77298-1-sven@svenpeter.dev>

On 2021-06-27 15:34, Sven Peter wrote:
[...]
> In the long term, I'd like to extend the dma-iommu framework itself to
> support iommu pagesizes with a larger granule than the CPU pagesize if that is
> something you agree with.

BTW this isn't something we can fully support in general. IOMMU API 
users may expect this to work:

iommu_map(domain, iova, page_to_phys(p1), PAGE_SIZE, prot);
iommu_map(domain, iova + PAGE_SIZE, page_to_phys(p2), PAGE_SIZE, prot);

Although they do in principle have visibility of pgsize_bitmap, I still 
doubt anyone is really prepared for CPU-page-aligned mappings to fail.
Even at the DMA API level you could hide *some* of it (at the cost of 
effectively only having 1/4 of the usable address space), but there are 
still cases like where v4l2 has a hard requirement that a page-aligned 
scatterlist can be mapped into a contiguous region of DMA addresses.

> This would be important to later support the thunderbolt DARTs since I would be
> very uncomfortable to have these running in (software or hardware) bypass mode.

Funnily enough that's the one case that would be relatively workable, 
since untrusted devices are currently subject to bounce-buffering of the 
entire DMA request, so it doesn't matter so much how the bounce buffer 
itself is mapped. Even with the possible future optimisation of only 
bouncing the non-page-aligned start and end parts of a buffer I think it 
still works (the physical alignment just has to be considered in terms 
of the IOMMU granule).

Robin.
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