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

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Date:   Mon, 3 Jun 2019 12:27:37 +0100
From:   Tom Murphy <tmurphy@...sta.com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     iommu@...ts.linux-foundation.org, Tom Murphy <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

On Mon, Jun 3, 2019 at 11:52 AM Joerg Roedel <joro@...tes.org> wrote:
>
> 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.

Where is the locking introduced? intel doesn't use a lock in it's
iommu_map function:
https://github.com/torvalds/linux/blob/f2c7c76c5d0a443053e94adb9f0918fa2fb85c3a/drivers/iommu/intel-iommu.c#L5302
because it cleverly uses cmpxchg64 to avoid using locks:
https://github.com/torvalds/linux/blob/f2c7c76c5d0a443053e94adb9f0918fa2fb85c3a/drivers/iommu/intel-iommu.c#L900
And the locking in AMD's iommu_map function can be removed (and i have
removed it in my patch set) because it does that same thing as intel:
https://github.com/torvalds/linux/blob/f2c7c76c5d0a443053e94adb9f0918fa2fb85c3a/drivers/iommu/amd_iommu.c#L1486

Is there something I'm missing?

>
> 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
>