On Sun, Jun 23, 2013 at 10:41:24PM -0600, Alex Williamson wrote: > On Mon, 2013-06-24 at 13:52 +1000, David Gibson wrote: > > On Sat, Jun 22, 2013 at 08:28:06AM -0600, Alex Williamson wrote: > > > On Sat, 2013-06-22 at 22:03 +1000, David Gibson wrote: > > > > On Thu, Jun 20, 2013 at 08:55:13AM -0600, Alex Williamson wrote: > > > > > On Thu, 2013-06-20 at 18:48 +1000, Alexey Kardashevskiy wrote: > > > > > > On 06/20/2013 05:47 PM, Benjamin Herrenschmidt wrote: > > > > > > > On Thu, 2013-06-20 at 15:28 +1000, David Gibson wrote: > > > > > > >>> Just out of curiosity - would not get_file() and fput_atomic() > > > > > > >>> on a > > > > > > >> group's > > > > > > >>> file* do the right job instead of > > > > > > >>> vfio_group_add_external_user() and > > > > > > >>> vfio_group_del_external_user()? > > > > > > >> > > > > > > >> I was thinking that too. Grabbing a file reference would > > > > > > >> certainly be > > > > > > >> the usual way of handling this sort of thing. > > > > > > > > > > > > > > But that wouldn't prevent the group ownership to be returned to > > > > > > > the kernel or another user would it ? > > > > > > > > > > > > > > > > > > Holding the file pointer does not let the group->container_users > > > > > > counter go > > > > > > to zero > > > > > > > > > > How so? Holding the file pointer means the file won't go away, which > > > > > means the group release function won't be called. That means the > > > > > group > > > > > won't go away, but that doesn't mean it's attached to an IOMMU. A > > > > > user > > > > > could call UNSET_CONTAINER. > > > > > > > > Uhh... *thinks*. Ah, I see. > > > > > > > > I think the interface should not take the group fd, but the container > > > > fd. Holding a reference to *that* would keep the necessary things > > > > around. But more to the point, it's the right thing semantically: > > > > > > > > The container is essentially the handle on a host iommu address space, > > > > and so that's what should be bound by the KVM call to a particular > > > > guest iommu address space. e.g. it would make no sense to bind two > > > > different groups to different guest iommu address spaces, if they were > > > > in the same container - the guest thinks they are different spaces, > > > > but if they're in the same container they must be the same space. > > > > > > While the container is the gateway to the iommu, what empowers the > > > container to maintain an iommu is the group. What happens to a > > > container when all the groups are disconnected or closed? Groups are > > > the unit that indicates hardware access, not containers. Thanks, > > > > Uh... huh? I'm really not sure what you're getting at. > > > > The operation we're doing for KVM here is binding a guest iommu > > address space to a particular host iommu address space. Why would we > > not want to use the obvious handle on the host iommu address space, > > which is the container fd? > > AIUI, the request isn't for an interface through which to do iommu > mappings. The request is for an interface to show that the user has > sufficient privileges to do mappings. Groups are what gives the user > that ability. The iommu is also possibly associated with multiple iommu > groups and I believe what is being asked for here is a way to hold and > lock a single iommu group with iommu protection. > > >From a practical point of view, the iommu interface is de-privileged > once the groups are disconnected or closed. Holding a reference count > on the iommu fd won't prevent that. That means we'd have to use a > notifier to have KVM stop the side-channel iommu access. Meanwhile > holding the file descriptor for the group and adding an interface that > bumps use counter allows KVM to lock itself in, just as if it had a > device opened itself. Thanks,
Ah, good point.
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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