On Wed, 9 Jun 2021 10:15:32 -0600 Alex Williamson <alex.williamson@xxxxxxxxxx> wrote: > On Wed, 9 Jun 2021 17:51:26 +0200 > Joerg Roedel <joro@xxxxxxxxxx> wrote: > > > On Wed, Jun 09, 2021 at 12:00:09PM -0300, Jason Gunthorpe wrote: > > > Only *drivers* know what the actual device is going to do, devices do > > > not. Since the group doesn't have drivers it is the wrong layer to be > > > making choices about how to configure the IOMMU. > > > > Groups don't carry how to configure IOMMUs, that information is > > mostly in the IOMMU domains. And those (or an abstraction of them) is > > configured through /dev/ioasid. So not sure what you wanted to say with > > the above. > > > > All a group carries is information about which devices are not > > sufficiently isolated from each other and thus need to always be in the > > same domain. > > > > > The device centric approach is my attempt at this, and it is pretty > > > clean, I think. > > > > Clean, but still insecure. > > > > > All ACS does is prevent P2P operations, if you assign all the group > > > devices into the same /dev/iommu then you may not care about that > > > security isolation property. At the very least it is policy for user > > > to decide, not kernel. > > > > It is a kernel decision, because a fundamental task of the kernel is to > > ensure isolation between user-space tasks as good as it can. And if a > > device assigned to one task can interfer with a device of another task > > (e.g. by sending P2P messages), then the promise of isolation is broken. > > AIUI, the IOASID model will still enforce IOMMU groups, but it's not an > explicit part of the interface like it is for vfio. For example the > IOASID model allows attaching individual devices such that we have > granularity to create per device IOASIDs, but all devices within an > IOMMU group are required to be attached to an IOASID before they can be > used. It's not entirely clear to me yet how that last bit gets > implemented though, ie. what barrier is in place to prevent device > usage prior to reaching this viable state. > > > > Groups should be primarily about isolation security, not about IOASID > > > matching. > > > > That doesn't make any sense, what do you mean by 'IOASID matching'? > > One of the problems with the vfio interface use of groups is that we > conflate the IOMMU group for both isolation and granularity. I think > what Jason is referring to here is that we still want groups to be the > basis of isolation, but we don't want a uAPI that presumes all devices > within the group must use the same IOASID. For example, if a user owns > an IOMMU group consisting of non-isolated functions of a multi-function > device, they should be able to create a vIOMMU VM where each of those > functions has its own address space. That can't be done today, the > entire group would need to be attached to the VM under a PCIe-to-PCI > bridge to reflect the address space limitation imposed by the vfio > group uAPI model. Thanks, Hmm, likely discussed previously in these threads, but I can't come up with the argument that prevents us from making the BIND interface at the group level but the ATTACH interface at the device level? For example: - VFIO_GROUP_BIND_IOASID_FD - VFIO_DEVICE_ATTACH_IOASID AFAICT that makes the group ownership more explicit but still allows the device level IOASID granularity. Logically this is just an internal iommu_group_for_each_dev() in the BIND ioctl. Thanks, Alex
