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

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

On Wed, 2015-10-28 at 22:31 +0900, David Woodhouse wrote:
> We have an option in the Intel IOMMU for pass-through mode too, which
> basically *is* a total bypass. In practice, what's the difference
> between that and a "simple translation that does not require any
> [translation]"? We set up a full 1:1 mapping of all memory, and then
> the map/unmap methods become no-ops.
> 
> Currently we have no way to request that mode on a per-device basis;
> we
> only have 'iommu=pt' on the command line to set it for *all* devices.
> But performance-sensitive devices might want it, while we keep doing
> proper translation for others.

On Power, I generally have 2 IOMMU windows for a device, one at the
bottom is remapped, and is generally used for 32-bit devices and the
one at the top us setup as a bypass (or in the case of KVM, as a linear
mapping of guest memory which looks the same as a bypass to the guest).

The DMA ops will automatically hit the appropriate one based on the
DMA mask.

I don't see how that attribute would work for us.

Cheers,
Ben.