Re: [v3 4/5] vfio: implement APIs to set/put kvm to/from vfio group

From: Paolo Bonzini <pbonzini@redhat.com>
To: Alex Williamson <alex.williamson@redhat.com>,
	Jike Song <jike.song@intel.com>
Cc: guangrong.xiao@linux.intel.com, kwankhede@nvidia.com,
	cjia@nvidia.com, kevin.tian@intel.com, kvm@vger.kernel.org
Subject: Re: [v3 4/5] vfio: implement APIs to set/put kvm to/from vfio group
Date: Mon, 7 Nov 2016 19:10:37 +0100	[thread overview]
Message-ID: <c94565d2-555d-fdd8-1131-4e7a316e2e4d@redhat.com> (raw)
In-Reply-To: <20161107110412.5db26fd4@t450s.home>

On 07/11/2016 19:04, Alex Williamson wrote:
>> > +struct kvm *vfio_group_get_kvm(struct vfio_group *group)
>> > +{
>> > +	struct kvm *kvm = NULL;
> Unnecessary initialization.
> 
>> > +
>> > +	mutex_lock(&group->udata.lock);
>> > +
>> > +	kvm = group->udata.kvm;
>> > +	if (kvm)
>> > +		kvm_get_kvm(kvm);
>> > +
>> > +	mutex_unlock(&group->udata.lock);
>> > +
>> > +	return kvm;
>> > +}
>> > +EXPORT_SYMBOL_GPL(vfio_group_get_kvm);
>
> How are kvm references acquired through vfio_group_get_kvm() ever
> released?

They are released with kvm_put_kvm, but it's done in the vendor driver
so that VFIO core doesn't have a dependency on kvm.ko.

> Can the reference become invalid?

No, this is guaranteed by virt/kvm/vfio.c + the udata.lock mutex (which
probably should be renamed...).

> The caller may still hold
> a kvm references, but couldn't the group be detached from one kvm
> instance and re-attached to another?

Can this be handled by the vendor driver?  Does it get a callback when
it's detached from a KVM instance?

Paolo