On Mon, 27 Oct 2014 17:25:02 +0100 Joerg Roedel <joro@xxxxxxxxxx> wrote: > On Mon, Oct 27, 2014 at 03:32:01PM +0100, Gerald Schaefer wrote: > > Not sure if I understood the concept of IOMMU domains right. But if > > this is about having multiple devices in the same domain, so that > > iommu_ops->map will establish the _same_ DMA mapping on _all_ > > registered devices, then this should be possible. > > Yes, this is what domains are about. A domain describes a set of DMA > mappings which can be assigned to multiple devices in parallel. > > > We cannot have shared DMA tables because each device gets its own > > DMA table allocated during device initialization. > > Is there some hardware reason for this or is that just an > implementation detail that can be changed. In other words, does the > hardware allow to use the same DMA table for multiple devices? Yes, the HW would allow shared DMA tables, but the implementation would need some non-trivial changes. For example, we have a per-device spin_lock for DMA table manipulations and the code in arch/s390/pci/pci_dma.c knows nothing about IOMMU domains or shared DMA tables, it just implements a set of dma_map_ops. Of course this would also go horribly wrong if a device was already in use (via the current dma_map_ops), but I guess using devices through the IOMMU_API prevents using them otherwise? > > > But we could just keep all devices from one domain in a list and > > then call dma_update_trans() for all devices during > > iommu_ops->map/unmap. > > This sounds complicated. Note that a device can be assigned to a > domain that already has existing mappings. In this case you need to > make sure that the new device inherits these mappings (and destroy > all old mappings for the device that possibly exist). > > I think it is much easier to use the same DMA table for all devices > in a domain, if the hardware allows that. Yes, in this case, having one DMA table per domain and sharing it between all devices in that domain sounds like a good idea. However, I can't think of any use case for this, and Frank probably had a very special use case in mind where this scenario doesn't appear, hence the "one device per domain" restriction. So, if having multiple devices per domain is a must, then we probably need a thorough rewrite of the arch/s390/pci/pci_dma.c code. Gerald -- To unsubscribe from this list: send the line "unsubscribe linux-s390" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
