On 23.08.2011, at 18:51, Benjamin Herrenschmidt wrote: > >>> For us the most simple and logical approach (which is also what pHyp >>> uses and what Linux handles well) is really to expose a given PCI host >>> bridge per group to the guest. Believe it or not, it makes things >>> easier :-) >> >> I'm all for easier. Why does exposing the bridge use less bus numbers >> than emulating a bridge? > > Because a host bridge doesn't look like a PCI to PCI bridge at all for > us. It's an entire separate domain with it's own bus number space > (unlike most x86 setups). > > In fact we have some problems afaik in qemu today with the concept of > PCI domains, for example, I think qemu has assumptions about a single > shared IO space domain which isn't true for us (each PCI host bridge > provides a distinct IO space domain starting at 0). We'll have to fix > that, but it's not a huge deal. > > So for each "group" we'd expose in the guest an entire separate PCI > domain space with its own IO, MMIO etc... spaces, handed off from a > single device-tree "host bridge" which doesn't itself appear in the > config space, doesn't need any emulation of any config space etc... > >> On x86, I want to maintain that our default assignment is at the device >> level. A user should be able to pick single or multiple devices from >> across several groups and have them all show up as individual, >> hotpluggable devices on bus 0 in the guest. Not surprisingly, we've >> also seen cases where users try to attach a bridge to the guest, >> assuming they'll get all the devices below the bridge, so I'd be in >> favor of making this "just work" if possible too, though we may have to >> prevent hotplug of those. >> >> Given the device requirement on x86 and since everything is a PCI device >> on x86, I'd like to keep a qemu command line something like -device >> vfio,host=00:19.0. I assume that some of the iommu properties, such as >> dma window size/address, will be query-able through an architecture >> specific (or general if possible) ioctl on the vfio group fd. I hope >> that will help the specification, but I don't fully understand what all >> remains. Thanks, > > Well, for iommu there's a couple of different issues here but yes, > basically on one side we'll have some kind of ioctl to know what segment > of the device(s) DMA address space is assigned to the group and we'll > need to represent that to the guest via a device-tree property in some > kind of "parent" node of all the devices in that group. > > We -might- be able to implement some kind of hotplug of individual > devices of a group under such a PHB (PCI Host Bridge), I don't know for > sure yet, some of that PAPR stuff is pretty arcane, but basically, for > all intend and purpose, we really want a group to be represented as a > PHB in the guest. > > We cannot arbitrary have individual devices of separate groups be > represented in the guest as siblings on a single simulated PCI bus. So would it make sense for you to go the same route that we need to go on embedded power, with a separate VFIO style interface that simply exports memory ranges and irq bindings, but doesn't know anything about PCI? For e500, we'll be using something like that to pass through a full PCI bus into the system. Alex -- To unsubscribe from this list: send the line "unsubscribe linux-pci" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
