linux-kernel - Re: [PATCH v3 0/4] iommu/amd: Convert the AMD iommu driver to the dma-iommu api

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

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