Re: [PATCH 3/8] vfio: add external user support

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

On Sun, 2013-07-07 at 01:07 +1000, Alexey Kardashevskiy wrote:
> VFIO is designed to be used via ioctls on file descriptors
> returned by VFIO.
> 
> However in some situations support for an external user is required.
> The first user is KVM on PPC64 (SPAPR TCE protocol) which is going to
> use the existing VFIO groups for exclusive access in real/virtual mode
> on a host to avoid passing map/unmap requests to the user space which
> would made things pretty slow.
> 
> The proposed protocol includes:
> 
> 1. do normal VFIO init stuff such as opening a new container, attaching
> group(s) to it, setting an IOMMU driver for a container. When IOMMU is
> set for a container, all groups in it are considered ready to use by
> an external user.
> 
> 2. pass a fd of the group we want to accelerate to KVM. KVM calls
> vfio_group_get_external_user() to verify if the group is initialized,
> IOMMU is set for it and increment the container user counter to prevent
> the VFIO group from disposal prior to KVM exit.
> The current TCE IOMMU driver marks the whole IOMMU table as busy when
> IOMMU is set for a container what prevents other DMA users from
> allocating from it so it is safe to grant user space access to it.
> 
> 3. KVM calls vfio_external_user_iommu_id() to obtian an IOMMU ID which
> KVM uses to get an iommu_group struct for later use.
> 
> 4. When KVM is finished, it calls vfio_group_put_external_user() to
> release the VFIO group by decrementing the container user counter.
> Everything gets released.
> 
> The "vfio: Limit group opens" patch is also required for the consistency.
> 
> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> ---
> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> index c488da5..57aa191 100644
> --- a/drivers/vfio/vfio.c
> +++ b/drivers/vfio/vfio.c
> @@ -1370,6 +1370,62 @@ static const struct file_operations vfio_device_fops = {
>  };
>  
>  /**
> + * External user API, exported by symbols to be linked dynamically.
> + *
> + * The protocol includes:
> + *  1. do normal VFIO init operation:
> + *	- opening a new container;
> + *	- attaching group(s) to it;
> + *	- setting an IOMMU driver for a container.
> + * When IOMMU is set for a container, all groups in it are
> + * considered ready to use by an external user.
> + *
> + * 2. The user space passed a group fd which we want to accelerate in
> + * KVM. KVM uses vfio_group_get_external_user() to verify that:
> + *	- the group is initialized;
> + *	- IOMMU is set for it.
> + * Then vfio_group_get_external_user() increments the container user
> + * counter to prevent the VFIO group from disposal prior to KVM exit.
> + *
> + * 3. KVM calls vfio_external_user_iommu_id() to know an IOMMU ID which
> + * KVM uses to get an iommu_group struct for later use.
> + *
> + * 4. When KVM is finished, it calls vfio_group_put_external_user() to
> + * release the VFIO group by decrementing the container user counter.

nit, the interface is for any external user, not just kvm.

> + */
> +struct vfio_group *vfio_group_get_external_user(struct file *filep)
> +{
> +	struct vfio_group *group = filep->private_data;
> +
> +	if (filep->f_op != &vfio_group_fops)
> +		return NULL;

ERR_PTR(-EINVAL)

There also needs to be a vfio_group_get(group) here and put in error
cases.

> +
> +	if (!atomic_inc_not_zero(&group->container_users))
> +		return NULL;

ERR_PTR(-EINVAL)

> +
> +	if (!group->container->iommu_driver ||
> +			!vfio_group_viable(group)) {
> +		atomic_dec(&group->container_users);
> +		return NULL;

ERR_PTR(-EINVAL)

> +	}
> +
> +	return group;
> +}
> +EXPORT_SYMBOL_GPL(vfio_group_get_external_user);
> +
> +void vfio_group_put_external_user(struct vfio_group *group)
> +{
> +	vfio_group_try_dissolve_container(group);

And a vfio_group_put(group) here

> +}
> +EXPORT_SYMBOL_GPL(vfio_group_put_external_user);
> +
> +int vfio_external_user_iommu_id(struct vfio_group *group)
> +{
> +	return iommu_group_id(group->iommu_group);
> +}
> +EXPORT_SYMBOL_GPL(vfio_external_user_iommu_id);
> +
> +/**
>   * Module/class support
>   */
>  static char *vfio_devnode(struct device *dev, umode_t *mode)
> diff --git a/include/linux/vfio.h b/include/linux/vfio.h
> index ac8d488..24579a0 100644
> --- a/include/linux/vfio.h
> +++ b/include/linux/vfio.h
> @@ -90,4 +90,11 @@ extern void vfio_unregister_iommu_driver(
>  	TYPE tmp;						\
>  	offsetof(TYPE, MEMBER) + sizeof(tmp.MEMBER); })		\
>  
> +/*
> + * External user API
> + */
> +extern struct vfio_group *vfio_group_get_external_user(struct file *filep);
> +extern void vfio_group_put_external_user(struct vfio_group *group);
> +extern int vfio_external_user_iommu_id(struct vfio_group *group);
> +
>  #endif /* VFIO_H */