linux-kernel - Re: [RFC PATCH] dma-iommu: Add iommu_dma_max_mapping

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Date:   Wed, 18 May 2022 14:45:07 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Christoph Hellwig <hch@....de>
Cc:     John Garry <john.garry@...wei.com>, joro@...tes.org,
        will@...nel.org, m.szyprowski@...sung.com,
        chenxiang66@...ilicon.com, thunder.leizhen@...wei.com,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        liyihang6@...ilicon.com
Subject: Re: [RFC PATCH] dma-iommu: Add iommu_dma_max_mapping_size()

On 2022-05-18 14:12, Christoph Hellwig wrote:
> On Tue, May 17, 2022 at 11:40:52AM +0100, Robin Murphy wrote:
>> Indeed, sorry but NAK for this being nonsense. As I've said at least once
>> before, if the unnecessary SAC address allocation attempt slows down your
>> workload, make it not do that in the first place. If you don't like the
>> existing command-line parameter then fine, there are plenty of other
>> options, it just needs to be done in a way that doesn't break x86 systems
>> with dodgy firmware, as my first attempt turned out to.
> 
> What broke x86?

See the thread at [1] (and in case of curiosity the other IVRS patches I 
refer to therein were at [2]). Basically, undescribed limitations lead 
to DMA address truncation once iommu-dma starts allocating from what it 
thinks is the full usable IOVA range. Your typical desktop PC is 
unlikely to have enough concurrent DMA-mapped memory to overflow the 
32-bit IOVA space naturally, so this has probably been hiding an untold 
multitude of sins over the years.

Robin.

[1] 
https://lore.kernel.org/linux-iommu/e583fc6dd1fb4ffc90310ff4372ee776f9cc7a3c.1594207679.git.robin.murphy@arm.com/
[2] 
https://lore.kernel.org/linux-iommu/20200605145655.13639-1-sebott@amazon.de/