Re: [PATCH v1 2/2] dma-mapping-common: add DMA attribute - DMA_ATTR_IOMMU_BYPASS

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Mon, 2015-11-02 at 22:48 +0000, David Woodhouse wrote:
> 
> In the Intel case, the mapping setup is entirely per-device (except for
> crap devices and devices behind a PCIe-PCI bridge, etc.).
> 
> So you can happily have a passthrough mapping for *one* device, without
> making that same mapping available to another device. You can make that
> trade-off of speed vs. protection for each device in the system.

Same for me. I should have written "in the context of a device ...",
the problem reamains that it doesn't buy you much to do it *per
-mapping* which is what this API seems to be about.

> Currently we have the 'iommu=pt' option that makes us use passthrough
> for *all* devices (except those whose dma_mask can't reach all of
> physical memory). But I could live with something like Shamir is
> proposing, which lets us do the bypass only for performance-sensitive
> devices which actually *ask* for it (and of course we'd have a system-
> wide mode which declines that request and does normal mappings anyway).
>