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