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Message-ID: <f88ea085-2161-02ff-ee36-a004b9e48338@arm.com>
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/
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