On Mon, Dec 19, 2011 at 11:11:25AM +1100, David Gibson wrote: > Well.. that's not where it is in Alex's code either. The iommu layer > (to the extent that there is such a "layer") supplies the group info, > but the group management is in vfio, not the iommu layer. With mine > it is in the driver core because the struct device seemed the logical > place for the group id.
Okay, seems we have different ideas of what the 'grouping code' is. I talked about the group enumeration code only. But group handling code is certainly important to some degree too. But before we argue about the right place of the code we should agree on the semantics such code should provide. For me it is fine when the code is in VFIO for now, since VFIO is the only user at the moment. When more users pop up we can easily move it out to somewhere else. But the semantics influence the interface to user-space too, so it is more important now. It splits up into a number of sub problems: 1) How userspace detects the device<->group relationship? 2) Do we want group-binding/unbinding to device drivers? 3) Group attach/detach to iommu-domains? 4) What to do with hot-added devices? For 1) I think the current solution with the iommu_group file is fine. It is somewhat expensive for user-space to figure out the per-group device-sets, but that is a one-time effort so it doesn't really matter. Probably we can rename 'iommu_group' to 'isolation_group' or something. Regarding 2), I think providing user-space a way to unbind groups of devices from their drivers is a horrible idea. It makes it too easy for the user to shoot himself in the foot. For example when the user wants to assign a network card to a guest, but that card is in the same group as the GPU and the screen wents blank when the guest is started. Requiring devices to be unbound one-by-one is better because this way the user always needs to know what he is doing. For the remaining two questions I think the concept of a default-domain is helpful. The default-domain is a per-group domain which is created by the iommu-driver at initialization time. It is the domain each device is assigned to when it is not assigned to any other domain (which means that each device/group is always attached to a domain). The default domain will be used by the DMA-API layer. This implicitly means, that a device which is not in the default-domain can't be used with the dma-api. The dma_supported() function will return false for those devices. So what does this mean for point 3? I think we can implement attaching and detaching groups in the iommu-api. This interface is not exposed to userspace and can help VFIO and possible future users. Semantic is, that domain_attach_group() only works when all devices in the group are in their default domain and domain_detach_group() puts them back into the default domain. Question 4) is also solved with the default-domain concept. A hot-added device is put in the domain of its group automatically. If the group is owned by VFIO and another driver attaches to the device dma_supported will return false and initialization will fail. > Right, so, the other problem is that a well boundaried "iommu-driver' > is something that only exists on x86 at present, and the "iommu api" > is riddled with x86-centric thinking. Or more accurately, design > based on how the current intel and amd iommus work. On systems like > POWER, use of the iommu is not optional - it's built into the PCI host > bridge and must be initialized when the bridge is probed, much earlier > than iommu driver initialization on x86. They have no inbuilt concept > of domains (though we could fake in software in some circumstances). Well, the iommu-api was designed for amd-vi and vt-d. But its concepts turn out to be more general and by no way x86-centric anymore. We support a couple of ARM platforms too for example. More to come. With small extensions to the API we will also support GART-like IOMMUs in the future. For your hardware the domain-concept will work too. In terms of the iommu-api a domain is nothing more than an address space. As far as I understand there is a 1-1 mapping between a hardware iommu and a domain in your case. The easiest solution then is to share the datastructures which describe the address space to the hardware between all iommus in a particular domain. Regards, Joerg -- AMD Operating System Research Center Advanced Micro Devices GmbH Einsteinring 24 85609 Dornach General Managers: Alberto Bozzo, Andrew Bowd Registration: Dornach, Landkr. Muenchen; Registerger. Muenchen, HRB Nr. 43632