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

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

On Mon, 31 Oct 2016 14:35:05 +0800
Jike Song <jike.song@intel.com> wrote:

Patch title is "set/put" but there is no "put".

> A vfio_group may be or may not be attached to a KVM instance,
> if it is, the user of vfio_group might also want to know which
> KVM instance it is attached to, to utilize features provided
> by KVM. In VFIO there are already external APIs for KVM to
> get/put the vfio_group, by extending that, KVM can set or clear
> itself to/from the vfio_group, for external users to use.
> 
> Signed-off-by: Jike Song <jike.song@intel.com>
> ---
>  drivers/vfio/vfio.c  | 30 ++++++++++++++++++++++++++++++
>  include/linux/vfio.h |  4 ++++
>  2 files changed, 34 insertions(+)
> 
> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> index e3e58e3..41611cc 100644
> --- a/drivers/vfio/vfio.c
> +++ b/drivers/vfio/vfio.c
> @@ -34,6 +34,7 @@
>  #include <linux/uaccess.h>
>  #include <linux/vfio.h>
>  #include <linux/wait.h>
> +#include <linux/kvm_host.h>
>  
>  #define DRIVER_VERSION	"0.3"
>  #define DRIVER_AUTHOR	"Alex Williamson <alex.williamson@redhat.com>"
> @@ -86,6 +87,10 @@ struct vfio_group {
>  	struct mutex			unbound_lock;
>  	atomic_t			opened;
>  	bool				noiommu;
> +	struct {
> +		struct kvm *kvm;
> +		struct mutex lock;
> +	} udata;
>  };
>  
>  struct vfio_device {
> @@ -333,6 +338,7 @@ static struct vfio_group *vfio_create_group(struct iommu_group *iommu_group)
>  	mutex_init(&group->device_lock);
>  	INIT_LIST_HEAD(&group->unbound_list);
>  	mutex_init(&group->unbound_lock);
> +	mutex_init(&group->udata.lock);
>  	atomic_set(&group->container_users, 0);
>  	atomic_set(&group->opened, 0);
>  	group->iommu_group = iommu_group;
> @@ -1739,6 +1745,30 @@ long vfio_external_check_extension(struct vfio_group *group, unsigned long arg)
>  }
>  EXPORT_SYMBOL_GPL(vfio_external_check_extension);
>  
> +void vfio_group_set_kvm(struct vfio_group *group, struct kvm *kvm)
> +{
> +	mutex_lock(&group->udata.lock);
> +	group->udata.kvm = kvm;
> +	mutex_unlock(&group->udata.lock);
> +}
> +EXPORT_SYMBOL_GPL(vfio_group_set_kvm);
> +
> +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?  Can the reference become invalid?  The caller may still hold
a kvm references, but couldn't the group be detached from one kvm
instance and re-attached to another?  This seems like an ad-hoc
reference that doesn't impose any usage semantics on the caller or
release mechanism.  Thanks,

Alex


>  /**
>   * Sub-module support
>   */
> diff --git a/include/linux/vfio.h b/include/linux/vfio.h
> index ad9b857..3abd690 100644
> --- a/include/linux/vfio.h
> +++ b/include/linux/vfio.h
> @@ -95,6 +95,10 @@ extern long vfio_external_check_extension(struct vfio_group *group,
>  extern struct vfio_group *vfio_group_get_from_dev(struct device *dev);
>  extern void vfio_group_put(struct vfio_group *group);
>  
> +struct kvm;
> +extern void vfio_group_set_kvm(struct vfio_group *group, struct kvm *kvm);
> +extern struct kvm *vfio_group_get_kvm(struct vfio_group *group);
> +
>  /*
>   * Sub-module helpers
>   */