Re: [PATCH 3/4] KVM: PPC: Add support for IOMMU in-kernel handling

[David Gibson]

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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