On Mon, 27 Oct 2014 18:58:35 +0100 Joerg Roedel <joro@xxxxxxxxxx> wrote: > On Mon, Oct 27, 2014 at 06:02:19PM +0100, Gerald Schaefer wrote: > > On Mon, 27 Oct 2014 17:25:02 +0100 > > Joerg Roedel <joro@xxxxxxxxxx> wrote: > > > 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. > > I think it would make sense to move the DMA table handling code and > the dma_map_ops implementation to the IOMMU driver too. This is also > how some other IOMMU drivers implement it. Yes, I feared that this would come up, but I agree that it looks like the best solution, at least if we really want/need the IOMMU API for s390 now. I'll need to discuss this with Frank, he seems to be on vacation this week. Thanks for your feedback and explanations! > The plan is to consolidate the dma_ops implementations someday and > have a common implementation that works with all IOMMU drivers across > architectures. This would benefit s390 as well and obsoletes the > driver specific dma_ops implementation. > > > 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? > > This is taken care of by the device drivers. A driver for a device > either uses the DMA-API or does its own management of DMA mappings > using the IOMMU-API. VFIO is an example for the later case. > > > > 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. > > One usecase is device access from user-space via VFIO. A userspace > process might want to access multiple devices at the same time and > VFIO would implement this by assigning all of these devices to the > same IOMMU domain. > > This requirement also comes also from the IOMMU-API itself. The > intention of the API is to make different IOMMUs look the same through > the API, and this is violated when drivers implement a 1-1 > domain->device mapping. > > > 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. > > Yes, this is a requirement for new IOMMU drivers. We already have > drivers implementing the same 1-1 relation and we are about to fix > them. But I don't want to add new drivers doing the same. > > > Joerg > > -- > 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 > -- 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
