On Thu, 2013-06-20 at 00:50 +1000, Benjamin Herrenschmidt wrote:
> On Wed, 2013-06-19 at 11:58 +0200, Alexander Graf wrote:
>
> > > Alex, any objection ?
> >
> > Which Alex? :)
>
> Heh, mostly Williamson in this specific case but your input is still
> welcome :-)
>
> > I think validate works, it keeps iteration logic out of the kernel
> > which is a good thing. There still needs to be an interface for
> > getting the iommu id in VFIO, but I suppose that one's for the other
> > Alex and Jörg to comment on.
>
> I think getting the iommu fd is already covered by separate patches from
> Alexey.
>
> > >
> > > Do we need to make it a get/put interface instead ?
> > >
> > > vfio_validate_and_use_iommu(file, iommu_id);
> > >
> > > vfio_release_iommu(file, iommu_id);
> > >
> > > To ensure that the resource remains owned by the process until KVM
> > > is closed as well ?
> > >
> > > Or do we want to register with VFIO with a callback so that VFIO can
> > > call us if it needs us to give it up ?
> >
> > Can't we just register a handler on the fd and get notified when it
> > closes? Can you kill VFIO access without closing the fd?
>
> That sounds actually harder :-)
>
> The question is basically: When we validate that relationship between a
> specific VFIO struct file with an iommu, what is the lifetime of that
> and how do we handle this lifetime properly.
>
> There's two ways for that sort of situation: The notification model
> where we get notified when the relationship is broken, and the refcount
> model where we become a "user" and thus delay the breaking of the
> relationship until we have been disposed of as well.
>
> In this specific case, it's hard to tell what is the right model from my
> perspective, which is why I would welcome Alex (W.) input.
>
> In the end, the solution will end up being in the form of APIs exposed
> by VFIO for use by KVM (via that symbol lookup mechanism) so Alex (W),
> as owner of VFIO at this stage, what do you want those to look
> like ? :-)
My first thought is that we should use the same reference counting as we
have for vfio devices (group->container_users). An interface for that
might look like:
int vfio_group_add_external_user(struct file *filep)
{
struct vfio_group *group = filep->private_data;
if (filep->f_op != &vfio_group_fops)
return -EINVAL;
if (!atomic_inc_not_zero(&group->container_users))
return -EINVAL;
return 0;
}
void vfio_group_del_external_user(struct file *filep)
{
struct vfio_group *group = filep->private_data;
BUG_ON(filep->f_op != &vfio_group_fops);
vfio_group_try_dissolve_container(group);
}
int vfio_group_iommu_id_from_file(struct file *filep)
{
struct vfio_group *group = filep->private_data;
BUG_ON(filep->f_op != &vfio_group_fops);
return iommu_group_id(group->iommu_group);
}
Would that work? Thanks,
Alex
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