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

[Arnd Bergmann <arnd@arndb.de>]

On Thursday 29 October 2015 10:10:46 Benjamin Herrenschmidt wrote:
> 
> > Maybe we should at least coordinate IOMMU 'paranoid/fast' modes across
> > architectures, and then the DMA_ATTR_IOMMU_BYPASS flag would have a
> > sane meaning in the paranoid mode (and perhaps we'd want an ultra
> > -paranoid mode where it's not honoured).
> 
> Possibly, though ideally that would be a user policy but of course by
> the time you get to userspace it's generally too late.

IIRC, we have an 'iommu=force' command line switch for this, to ensure
that no device can use a linear mapping and everything goes through
the page tables. This is often useful for both debugging and as a
security measure when dealing with unpriviledged DMA access (virtual
machines, vfio, ...).

If we add a DMA_ATTR_IOMMU_BYPASS attribute, we should clearly document
which of the two we expect to take priority in cases where we have a
choice.

I wonder if the 'iommu=force' attribute is too coarse-grained though,
and if we should perhaps allow a per-device setting on architectures
that allow this.

	Arnd