Re: [RFC 13/20] iommu: Extend iommu_at[de]tach_device() for multiple devices group

David Gibson
2021-10-14T06:22:50Z

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On Sun, Sep 19, 2021 at 02:38:41PM +0800, Liu Yi L wrote:
> From: Lu Baolu <baolu.lu@linux.intel.com>
>
> These two helpers could be used when 1) the iommu group is singleton,
> or 2) the upper layer has put the iommu group into the secure state by
> calling iommu_device_init_user_dma().
>
> As we want the iommufd design to be a device-centric model, we want to
> remove any group knowledge in iommufd. Given that we already have
> iommu_at[de]tach_device() interface, we could extend it for iommufd
> simply by doing below:
>
> - first device in a group does group attach;
> - last device in a group does group detach.
>
> as long as the group has been put into the secure context.
>
> The commit <426a273834eae> ("iommu: Limit iommu_attach/detach_device to
> device with their own group") deliberately restricts the two interfaces
> to single-device group. To avoid the conflict with existing usages, we
> keep this policy and put the new extension only when the group has been
> marked for user_dma.

I still kind of hate this interface because it means an operation that
appears to be explicitly on a single device has an implicit effect on
other devices.

> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
> ---
> drivers/iommu/iommu.c | 25 +++++++++++++++++++++----
> 1 file changed, 21 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index bffd84e978fb..b6178997aef1 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -47,6 +47,7 @@ struct iommu_group {
> struct list_head entry;
> unsigned long user_dma_owner_id;
> refcount_t owner_cnt;
> + refcount_t attach_cnt;
> };
>
> struct group_device {
> @@ -1994,7 +1995,7 @@ static int __iommu_attach_device(struct iommu_domain *domain,
> int iommu_attach_device(struct iommu_domain *domain, struct device *dev)
> {
> struct iommu_group *group;
> - int ret;
> + int ret = 0;
>
> group = iommu_group_get(dev);
> if (!group)
> @@ -2005,11 +2006,23 @@ int iommu_attach_device(struct iommu_domain *domain, struct device *dev)
> * change while we are attaching
> */
> mutex_lock(&group->mutex);
> - ret = -EINVAL;
> - if (iommu_group_device_count(group) != 1)
> + if (group->user_dma_owner_id) {
> + if (group->domain) {
> + if (group->domain != domain)
> + ret = -EBUSY;
> + else
> + refcount_inc(&group->attach_cnt);
> +
> + goto out_unlock;
> + }
> + } else if (iommu_group_device_count(group) != 1) {

With this condition in the else, how can you ever attach the first
device of a multi-device group?

> + ret = -EINVAL;
> goto out_unlock;
> + }
>
> ret = __iommu_attach_group(domain, group);
> + if (!ret && group->user_dma_owner_id)
> + refcount_set(&group->attach_cnt, 1);
>
> out_unlock:
> mutex_unlock(&group->mutex);
> @@ -2261,7 +2274,10 @@ void iommu_detach_device(struct iommu_domain *domain, struct device *dev)
> return;
>
> mutex_lock(&group->mutex);
> - if (iommu_group_device_count(group) != 1) {
> + if (group->user_dma_owner_id) {
> + if (!refcount_dec_and_test(&group->attach_cnt))
> + goto out_unlock;
> + } else if (iommu_group_device_count(group) != 1) {

Shouldn't this path (detach a thing that's not attached), be a no-op
regardless of whether it's a singleton group or not? Why does one
deserve a warning and one not?

> WARN_ON(1);
> goto out_unlock;
> }
> @@ -3368,6 +3384,7 @@ static int iommu_group_init_user_dma(struct iommu_group *group,
>
> group->user_dma_owner_id = owner;
> refcount_set(&group->owner_cnt, 1);
> + refcount_set(&group->attach_cnt, 0);
>
> /* default domain is unsafe for user-initiated dma */
> if (group->domain == group->default_domain)

--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson

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