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From:   Tom Murphy <tmurphy@arista.com>
Date:   Mon, 3 Jun 2019 12:27:37 +0100
Message-ID: <CAPL0++72dekt=re1=sTWpCJtMX=mUOc3Jcq=9d1sr1QO25_zFA@mail.gmail.com>
Subject: Re: [PATCH v3 0/4] iommu/amd: Convert the AMD iommu driver to the
 dma-iommu api
To:     Joerg Roedel <joro@8bytes.org>
Cc:     iommu@lists.linux-foundation.org, Tom Murphy <murphyt7@tcd.ie>,
        Will Deacon <will.deacon@arm.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
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        David Brown <david.brown@linaro.org>,
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On Mon, Jun 3, 2019 at 11:52 AM Joerg Roedel <joro@8bytes.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
>