Re: [v6 2/3] vfio: support notifier chain in vfio_group

[Alex Williamson <alex.williamson@redhat.com>]

On Sat, 19 Nov 2016 12:14:29 +0800
Jike Song <jike.song@intel.com> wrote:

> On 11/19/2016 01:55 AM, Alex Williamson wrote:
> > On Fri, 18 Nov 2016 19:01:48 +0800
> > Jike Song <jike.song@intel.com> wrote:  
> >> Beyond vfio_iommu events, users might also be interested in
> >> vfio_group events. For example, if a vfio_group is used along
> >> with Qemu/KVM, whenever kvm pointer is set to/cleared from the
> >> vfio_group, users could be notified.
> >>
> >> Currently only VFIO_GROUP_NOTIFY_SET_KVM supported.
> >>
> >> Cc: Alex Williamson <alex.williamson@redhat.com>
> >> Cc: Kirti Wankhede <kwankhede@nvidia.com>
> >> Cc: Paolo Bonzini <pbonzini@redhat.com>
> >> Cc: Xiao Guangrong <guangrong.xiao@linux.intel.com>
> >> Signed-off-by: Jike Song <jike.song@intel.com>
> >> ---
> >>  drivers/vfio/vfio.c  | 69 ++++++++++++++++++++++++++++++++++++++++++++++++++++
> >>  include/linux/vfio.h |  6 +++++
> >>  2 files changed, 75 insertions(+)
> >>
> >> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> >> index ec62bec..e2bb197 100644
> >> --- a/drivers/vfio/vfio.c
> >> +++ b/drivers/vfio/vfio.c
> >> @@ -86,6 +86,8 @@ struct vfio_group {
> >>  	struct mutex			unbound_lock;
> >>  	atomic_t			opened;
> >>  	bool				noiommu;
> >> +	struct kvm			*kvm;
> >> +	struct blocking_notifier_head	notifier;
> >>  };
> >>  
> >>  struct vfio_device {
> >> @@ -339,6 +341,7 @@ static struct vfio_group *vfio_create_group(struct iommu_group *iommu_group)
> >>  #ifdef CONFIG_VFIO_NOIOMMU
> >>  	group->noiommu = (iommu_group_get_iommudata(iommu_group) == &noiommu);
> >>  #endif
> >> +	BLOCKING_INIT_NOTIFIER_HEAD(&group->notifier);
> >>  
> >>  	group->nb.notifier_call = vfio_iommu_group_notifier;
> >>  
> >> @@ -1015,6 +1018,63 @@ static long vfio_ioctl_check_extension(struct vfio_container *container,
> >>  	return ret;
> >>  }
> >>  
> >> +void vfio_group_set_kvm(struct vfio_group *group, struct kvm *kvm)
> >> +{
> >> +	group->kvm = kvm;
> >> +	blocking_notifier_call_chain(&group->notifier,
> >> +				VFIO_GROUP_NOTIFY_SET_KVM, kvm);
> >> +}
> >> +EXPORT_SYMBOL_GPL(vfio_group_set_kvm);
> >> +
> >> +static int vfio_register_group_notifier(struct vfio_group *group,
> >> +					unsigned long *events,
> >> +					struct notifier_block *nb)
> >> +{
> >> +	int ret;
> >> +	bool set_kvm = false;
> >> +
> >> +	if (*events & VFIO_GROUP_NOTIFY_SET_KVM)
> >> +		set_kvm = true;
> >> +
> >> +	/* clear known events */
> >> +	*events &= ~VFIO_GROUP_NOTIFY_SET_KVM;
> >> +
> >> +	/* refuse to continue if still events remaining */
> >> +	if (*events)
> >> +		return -EINVAL;
> >> +
> >> +	if (!atomic_inc_not_zero(&group->opened))
> >> +		return -EINVAL;  
> > 
> > vfio_group.opened is only used to make sure we don't allow multiple
> > opens of the group, incrementing it doesn't currently assure the group
> > remains opened.  What happens if the user process releases the group in
> > the midst of this?  
> 
> Thanks for pointing out this.
> It seems okay to think the group is open by checking group->opened,
> but after that I failed to find any existing method to prevent a concurrent
> vfio_group_fops_release, could you enlighten me a bit?

I don't think we have such a thing.  I briefly investigated whether we
should add a group mutex rather than the atomic, but at this point I'm
just leaning towards using the same conditions as attaching the iommu
notifier, ie. call vfio_group_add_container_user().  This is also what
we do for vfio_group_get_external_user() so I think it makes sense that
any sort of group reference or notifier registration have the same
requirements.  Thanks,

Alex