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Date: Mon, 3 Jun 2019 12:51:58 +0200
From: Joerg Roedel <joro@8bytes.org>
Subject: Re: [PATCH v3 0/4] iommu/amd: Convert the AMD iommu driver to the
 dma-iommu api
Message-ID: <20190603105158.GL12745@8bytes.org>
References: <20190506185207.31069-1-tmurphy@arista.com>
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To: Tom Murphy <tmurphy@arista.com>
Cc: iommu@lists.linux-foundation.org, murphyt7@tcd.ie, Will Deacon <will.deacon@arm.com>, Robin Murphy <robin.murphy@arm.com>, Marek Szyprowski <m.szyprowski@samsung.com>, Kukjin Kim <kgene@kernel.org>, Krzysztof Kozlowski <krzk@kernel.org>, David Woodhouse <dwmw2@infradead.org>, Andy Gross <andy.gross@linaro.org>, David Brown <david.brown@linaro.org>, Matthias Brugger <matthias.bgg@gmail.com>, Rob Clark <robdclark@gmail.com>, Heiko Stuebner <heiko@sntech.de>, Gerald Schaefer <gerald.schaefer@de.ibm.com>, Thierry Reding <thierry.reding@gmail.com>, Jonathan Hunter <jonathanh@nvidia.com>, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-samsung-soc@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-mediatek@lists.infradead.org, linux-rockchip@lists.infradead.org, linux-s390@vger.kernel.org, linux-tegra@vger.kernel.org

Hi Tom,

On Mon, May 06, 2019 at 07:52:02PM +0100, Tom Murphy wrote:
> Convert the AMD iommu driver to the dma-iommu api. Remove the iova
> handling and reserve region code from the AMD iommu driver.

Thank you for your work on this! I appreciate that much, but I am not
sure we are ready to make that move for the AMD and Intel IOMMU drivers
yet.

My main concern right now is that these changes will add a per-page
table lock into the fast-path for dma-mapping operations. There has been
much work in the past to remove all locking from these code-paths and
make it scalable on x86.

The dma-ops implementations in the x86 IOMMU drivers have the benefit
that they can call their page-table manipulation functions directly and
without locks, because they can make the necessary assumptions. The
IOMMU-API mapping/unmapping path can't make these assumptions because it
is also used for non-DMA-API use-cases.

So before we can move the AMD and Intel drivers to the generic DMA-API
implementation we need to solve this problem to not introduce new
scalability regressions.

Regards,

	Joerg