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Message-ID: <f88ea085-2161-02ff-ee36-a004b9e48338@arm.com>
Date:   Wed, 18 May 2022 14:45:07 +0100
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Subject: Re: [RFC PATCH] dma-iommu: Add iommu_dma_max_mapping_size()
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To:     Christoph Hellwig <hch@lst.de>
Cc:     John Garry <john.garry@huawei.com>, joro@8bytes.org,
        will@kernel.org, m.szyprowski@samsung.com,
        chenxiang66@hisilicon.com, thunder.leizhen@huawei.com,
        iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org,
        liyihang6@hisilicon.com
References: <1652706361-92557-1-git-send-email-john.garry@huawei.com>
 <f5b78c9c-312e-70ab-ecbb-f14623a4b6e3@arm.com>
 <20220518131237.GA26019@lst.de>
From:   Robin Murphy <robin.murphy@arm.com>
In-Reply-To: <20220518131237.GA26019@lst.de>
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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/