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

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

On Fri, 2015-10-30 at 11:32 +0100, Arnd Bergmann wrote:
> 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 th ough
> the page tables. This is often useful for both debugging and as a
> security measure when dealing with unpriviledged DMA access (virtual
> machines, vfio, ...).

That was used to force-enable the iommu on platforms like G5s where we
would otherwise only do so if the memory was larger than 32-bit but we
never implemented using it to prevent the bypass region.

> If we add a DMA_ATTR_IOMMU_BYPASS attribute, we should clearly
> document
> which osed to force-enable the iommu on HGthe 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.

The interesting thing, if we can make it work, is the bypass attribute
being per mapping... 

Ben.