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. -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
