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SubjectRe: NVMe vs DMA addressing limitations

>> Another workaround me might need is to limit amount of concurrent DMA
>> in the NVMe driver based on some platform quirk. The way that NVMe works,
>> it can have very large amounts of data that is concurrently mapped into
>> the device.
> That's not really just NVMe - other storage and network controllers also
> can DMA map giant amounts of memory. There are a couple aspects to it:
> - dma coherent memoery - right now NVMe doesn't use too much of it,
> but upcoming low-end NVMe controllers will soon start to require
> fairl large amounts of it for the host memory buffer feature that
> allows for DRAM-less controller designs. As an interesting quirk
> that is memory only used by the PCIe devices, and never accessed
> by the Linux host at all.

Would it make sense to convert the nvme driver to use normal allocations
and use the DMA streaming APIs (dma_sync_single_for_[cpu|device]) for
both queues and future HMB?

> - size vs number of the dynamic mapping. We probably want the dma_ops
> specify a maximum mapping size for a given device. As long as we
> can make progress with a few mappings swiotlb / the iommu can just
> fail mapping and the driver will propagate that to the block layer
> that throttles I/O.

Isn't max mapping size per device too restrictive? it is possible that
not all devices posses active mappings concurrently.

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