Re: [RFC PATCH] dma-iommu: Add iommu_dma_max_mapping_size()

[Robin Murphy]

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@xxxxxxx/
[2] 
https://lore.kernel.org/linux-iommu/20200605145655.13639-1-sebott@xxxxxxxxx/