On Tue, Aug 02, 2011 at 09:34:58AM -0600, Alex Williamson wrote: > On Tue, 2011-08-02 at 22:58 +1000, Benjamin Herrenschmidt wrote: > > > > Don't worry, it took me a while to get my head around the HW :-) SR-IOV > > VFs will generally not have limitations like that no, but on the other > > hand, they -will- still require 1 VF = 1 group, ie, you won't be able to > > take a bunch of VFs and put them in the same 'domain'. > > > > I think the main deal is that VFIO/qemu sees "domains" as "guests" and > > tries to put all devices for a given guest into a "domain". > > Actually, that's only a recent optimization, before that each device got > it's own iommu domain. It's actually completely configurable on the > qemu command line which devices get their own iommu and which share. > The default optimizes the number of domains (one) and thus the number of > mapping callbacks since we pin the entire guest. > > > On POWER, we have a different view of things were domains/groups are > > defined to be the smallest granularity we can (down to a single VF) and > > we give several groups to a guest (ie we avoid sharing the iommu in most > > cases) > > > > This is driven by the HW design but that design is itself driven by the > > idea that the domains/group are also error isolation groups and we don't > > want to take all of the IOs of a guest down if one adapter in that guest > > is having an error. > > > > The x86 domains are conceptually different as they are about sharing the > > iommu page tables with the clear long term intent of then sharing those > > page tables with the guest CPU own. We aren't going in that direction > > (at this point at least) on POWER.. > > Yes and no. The x86 domains are pretty flexible and used a few > different ways. On the host we do dynamic DMA with a domain per device, > mapping only the inflight DMA ranges. In order to achieve the > transparent device assignment model, we have to flip that around and map > the entire guest. As noted, we can continue to use separate domains for > this, but since each maps the entire guest, it doesn't add a lot of > value and uses more resources and requires more mapping callbacks (and > x86 doesn't have the best error containment anyway). If we had a well > supported IOMMU model that we could adapt for pvDMA, then it would make > sense to keep each device in it's own domain again. Thanks, Could you have an PV IOMMU (in the guest) that would set up those maps? -- 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
