On Wed, Apr 01, 2015 at 04:38:54PM +0100, Joerg Roedel wrote: > Hi Will,
Hi Joerg, > On Tue, Mar 31, 2015 at 03:49:56PM +0100, Will Deacon wrote: > > But isn't this restriction already the case in practice? For example, if > > I have a domain with some mapping already configured, then that mapping > > will be using some fixed set of page sizes. Attaching a device behind > > another IOMMU that doesn't support that page size would effectively require > > the domain page tables to be freed and re-allocated from scratch. > > The problem is that this restriction depends on the IOMMU driver in use. > From the beginning of the IOMMU-API the backend drivers have supported > sharing a domain by multiple devices. In fact, the domain is just an > abstraction for an address space that can be attached to devices just > like cpu page-tables are assigned to threads of a process. I think the domain is a useful abstraction; I just don't think it works too well when you attach devices upstream of different IOMMUs to the same domain. If all the devices attached to a domain share an IOMMU, everything works nicely. > We can discuss whether this fundamental concept of the IOMMU-API needs > to be changed (moving into the direction this patch-set proposes). I > could imaging removing the domain concept entirely and just have > map/unmap functions like this: > > iommu_map(struct device *dev, dma_addr_t iova, > phys_addr_t phys, size_t size) > iommu_unmap(struct device *dev, dma_addr_t iova, > phys_addr_t phys, size_t size) > > This would require some changes for the users of the IOMMU-API, but it > shouldn't be too hard. > > Or we keep the desired semantics as they are now and do everything > needed for that in the IOMMU drivers. For the arm-smmu this would mean > exposing a common set of supported pgsizes between IOMMUs, or to build > multiple page-tables to match the different IOMMU capabilities. > > I am open for disussions for either way, but I like the current > semantics a bit more, as it allows us to share page-tables between > devices and we can move all of the nasty code in VFIO that already > creates multiple domains to get different page-tables into the > IOMMU core or the drivers (were it belongs). I agree that sharing page tables is desirable and I certainly wouldn't suggest removing the domain abstraction. > What I definitly don't want is a mixture of both concepts depending on > the IOMMU driver in use. We should settle on one way and force the > drivers to behave accordingly. We don't need a common API when every > driver behaves differently. Agreed. How would you feel about restricting domains to be per-IOMMU instance? VFIO already copes with this, so I think we'd just need something to keep legacy KVM device passthrough working on x86. Maybe we could add a new domain type using your new series (DOMAIN_X86_KVM_LEGACY or something) and avoid the cross-IOMMU domain checks for that. Will _______________________________________________ iommu mailing list [email protected] https://lists.linuxfoundation.org/mailman/listinfo/iommu
