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? -- 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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