NVMe vs DMA addressing limitations

Christoph Hellwig
2017-01-10T14:48:39Z

On Tue, Jan 10, 2017 at 12:01:05PM +0100, Arnd Bergmann wrote:
> 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.

- 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.