Re: [PATCH 2/5] iommu: Allocate a default domain for iommu groups

[Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>]

On Tue, Jan 27, 2015 at 12:08:56AM +0000, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel-l3A5Bk7waGM@public.gmane.org>
> 
> The default domain will be used (if supported by the iommu
> driver) when the devices in the iommu group are not attached
> to any other domain.
> 
> Signed-off-by: Joerg Roedel <jroedel-l3A5Bk7waGM@public.gmane.org>
> ---
>  drivers/iommu/iommu.c | 26 +++++++++++++++++++++++---
>  1 file changed, 23 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 34636eb..8f33ddd3 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -76,6 +76,9 @@ struct iommu_group_attribute iommu_group_attr_##_name =		\
>  #define to_iommu_group(_kobj)		\
>  	container_of(_kobj, struct iommu_group, kobj)
>  
> +static struct iommu_domain *__iommu_domain_alloc(struct bus_type *bus,
> +						 enum iommu_domain_type type);
> +
>  static ssize_t iommu_group_attr_show(struct kobject *kobj,
>  				     struct attribute *__attr, char *buf)
>  {
> @@ -362,6 +365,17 @@ rename:
>  
>  	kobject_get(group->devices_kobj);
>  
> +	/*
> +	 * Try to allocate a default domain for the group, if this
> +	 * hasn't happened yet. This is not the best place to do that,
> +	 * it should happen in iommu_group_alloc(). But we have no
> +	 * iommu_ops there yet, so the allocation has to happen here for
> +	 * now.
> +	 */
> +	if (group->default_domain == NULL)
> +		group->default_domain = __iommu_domain_alloc(dev->bus,
> +							     IOMMU_DOMAIN_DMA);

Having a per-group domain is wasteful for IOMMUs that only support a modest
number of concurrent domains, so in reality I think we need to have one
domain per IOMMU instance. Is that possible?

One problem with the current per-bus approach is that __iommu_domain_alloc
can't figure out which IOMMU instance corresponds to the group.

Will