Presently, the default domain of an iommu group is allocated during boot
time and it cannot be changed later. So, the device would typically be
either in identity (also known as pass_through) mode or the device would be
in DMA mode as long as the machine is up and running. There is no way to
change the default domain type dynamically i.e. after booting, a device
cannot switch between identity mode and DMA mode.

But, assume a use case wherein the user trusts the device and believes that
the OS is secure enough and hence wants *only* this device to bypass IOMMU
(so that it could be high performing) whereas all the other devices to go
through IOMMU (so that the system is protected). Presently, this use case
is not supported. It will be helpful if there is some way to change the
default domain of an iommu group dynamically. Hence, add such support.

A privileged user could request the kernel to change the default domain
type of a iommu group by writing to
"/sys/kernel/iommu_groups/<grp_id>/type" file. Presently, only three values
are supported
1. identity: all the DMA transactions from the device in this group are
             *not* translated by the iommu
2. DMA: all the DMA transactions from the device in this group are
        translated by the iommu
3. auto: change to the type the device was booted with

Note:
1. Default domain of an iommu group with two or more devices cannot be
   changed.
2. The device in the iommu group shouldn't be bound to any driver.
3. The device shouldn't be assigned to user for direct access.
4. The vendor iommu driver is required to add def_domain_type() callback.
   The change request will fail if the request type conflicts with that
   returned from the callback.

Please see "Documentation/ABI/testing/sysfs-kernel-iommu_groups" for more
information.

Cc: Christoph Hellwig <h...@lst.de>
Cc: Joerg Roedel <j...@8bytes.org>
Cc: Ashok Raj <ashok....@intel.com>
Cc: Will Deacon <will.dea...@arm.com>
Cc: Lu Baolu <baolu...@linux.intel.com>
Cc: Sohil Mehta <sohil.me...@intel.com>
Cc: Robin Murphy <robin.mur...@arm.com>
Cc: Jacob Pan <jacob.jun....@linux.intel.com>
Signed-off-by: Sai Praneeth Prakhya <sai.praneeth.prak...@intel.com>
---
 drivers/iommu/iommu.c | 215 +++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 214 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index d43120eb1dc5..b023f06f12d6 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -93,6 +93,8 @@ static void __iommu_detach_group(struct iommu_domain *domain,
 static int iommu_create_device_direct_mappings(struct iommu_group *group,
                                               struct device *dev);
 static struct iommu_group *iommu_group_get_for_dev(struct device *dev);
+static ssize_t iommu_group_store_type(struct iommu_group *group,
+                                     const char *buf, size_t count);
 
 #define IOMMU_GROUP_ATTR(_name, _mode, _show, _store)          \
 struct iommu_group_attribute iommu_group_attr_##_name =                \
@@ -525,7 +527,8 @@ static IOMMU_GROUP_ATTR(name, S_IRUGO, 
iommu_group_show_name, NULL);
 static IOMMU_GROUP_ATTR(reserved_regions, 0444,
                        iommu_group_show_resv_regions, NULL);
 
-static IOMMU_GROUP_ATTR(type, 0444, iommu_group_show_type, NULL);
+static IOMMU_GROUP_ATTR(type, 0644, iommu_group_show_type,
+                       iommu_group_store_type);
 
 static void iommu_group_release(struct kobject *kobj)
 {
@@ -2838,3 +2841,213 @@ int iommu_sva_get_pasid(struct iommu_sva *handle)
        return ops->sva_get_pasid(handle);
 }
 EXPORT_SYMBOL_GPL(iommu_sva_get_pasid);
+
+/*
+ * Changes the default domain of an iommu group
+ *
+ * @group: The group for which the default domain should be changed
+ * @type: The type of the new default domain that gets associated with the 
group
+ *
+ * Returns 0 on success and error code on failure
+ *
+ * Note:
+ * 1. Presently, this function is called only when user requests to change the
+ *    group's default domain type through 
/sys/kernel/iommu_groups/<grp_id>/type
+ *    Please take a closer look if intended to use for other purposes.
+ */
+static int iommu_change_dev_def_domain(struct iommu_group *group, int type)
+{
+       struct iommu_domain *prev_dom;
+       struct group_device *grp_dev;
+       const struct iommu_ops *ops;
+       int ret, dev_def_dom;
+       struct device *dev;
+
+       if (!group)
+               return -EINVAL;
+
+       mutex_lock(&group->mutex);
+
+       if (group->default_domain != group->domain) {
+               pr_err_ratelimited("Group assigned to user level for direct 
access\n");
+               ret = -EBUSY;
+               goto out;
+       }
+
+       /*
+        * iommu group wasn't locked while acquiring device lock in
+        * iommu_group_store_type(). So, make sure that the device count hasn't
+        * changed while acquiring device lock.
+        *
+        * Changing default domain of an iommu group with two or more devices
+        * isn't supported because there could be a potential deadlock. Consider
+        * the following scenario. T1 is trying to acquire device locks of all
+        * the devices in the group and before it could acquire all of them,
+        * there could be another thread T2 (from different sub-system and use
+        * case) that has already acquired some of the device locks and might be
+        * waiting for T1 to release other device locks.
+        */
+       if (iommu_group_device_count(group) != 1) {
+               pr_err_ratelimited("Cannot change default domain of a group 
with two or more devices\n");
+               ret = -EINVAL;
+               goto out;
+       }
+
+       /* Since group has only one device */
+       list_for_each_entry(grp_dev, &group->devices, list)
+               dev = grp_dev->dev;
+
+       prev_dom = group->default_domain;
+       if (!prev_dom || !prev_dom->ops || !prev_dom->ops->def_domain_type) {
+               pr_err_ratelimited("'def_domain_type' call back isn't 
registered\n");
+               ret = -EINVAL;
+               goto out;
+       }
+
+       ops = prev_dom->ops;
+       dev_def_dom = ops->def_domain_type(dev);
+
+       /* Check if user requested domain is supported by the device or not */
+       if (!type) {
+               /*
+                * If the user hasn't requested any specific type of domain and
+                * if the device supports both the domains, then default to the
+                * domain the device was booted with
+                */
+               type = dev_def_dom ?: iommu_def_domain_type;
+       } else if (dev_def_dom && type != dev_def_dom) {
+               pr_err_ratelimited("Device cannot be in %s domain\n",
+                                  iommu_domain_type_str(type));
+               ret = -EINVAL;
+               goto out;
+       }
+
+       /*
+        * Switch to a new domain only if the requested domain type is different
+        * from the existing default domain type
+        */
+       if (prev_dom->type == type) {
+               ret = 0;
+               goto out;
+       }
+
+       /* Sets group->default_domain to the newly allocated domain */
+       ret = iommu_group_alloc_default_domain(dev->bus, group, type);
+       if (ret)
+               goto out;
+
+       ret = __iommu_attach_device(group->default_domain, dev);
+       if (ret)
+               goto free_new_domain;
+
+       group->domain = group->default_domain;
+
+       ret = iommu_create_device_direct_mappings(group, dev);
+       if (ret)
+               goto free_new_domain;
+
+       /*
+        * Release the mutex here because ops->probe_finalize() call-back of
+        * some vendor IOMMU drivers calls arm_iommu_attach_device() which
+        * in-turn might call back into IOMMU core code, where it tries to take
+        * group->mutex, resulting in a deadlock.
+        */
+       mutex_unlock(&group->mutex);
+
+       /* Make sure dma_ops is appropriatley set */
+       iommu_group_do_probe_finalize(dev, group->default_domain);
+       iommu_domain_free(prev_dom);
+       return 0;
+
+free_new_domain:
+       iommu_domain_free(group->default_domain);
+       group->default_domain = prev_dom;
+       group->domain = prev_dom;
+
+out:
+       mutex_unlock(&group->mutex);
+
+       return ret;
+}
+
+static ssize_t iommu_group_store_type(struct iommu_group *group,
+                                     const char *buf, size_t count)
+{
+       struct group_device *grp_dev;
+       struct device *dev;
+       int ret, req_type;
+
+       if (!capable(CAP_SYS_ADMIN) || !capable(CAP_SYS_RAWIO))
+               return -EACCES;
+
+       if (WARN_ON(!group))
+               return -EINVAL;
+
+       if (sysfs_streq(buf, "identity"))
+               req_type = IOMMU_DOMAIN_IDENTITY;
+       else if (sysfs_streq(buf, "DMA"))
+               req_type = IOMMU_DOMAIN_DMA;
+       else if (sysfs_streq(buf, "auto"))
+               req_type = 0;
+       else
+               return -EINVAL;
+
+       /*
+        * Lock/Unlock the group mutex here before device lock to
+        * 1. Make sure that the iommu group has only one device (this is a
+        *    prerequisite for step 2)
+        * 2. Get struct *dev which is needed to lock device
+        */
+       mutex_lock(&group->mutex);
+       if (iommu_group_device_count(group) != 1) {
+               mutex_unlock(&group->mutex);
+               pr_err_ratelimited("Cannot change default domain of a group 
with two or more devices\n");
+               return -EINVAL;
+       }
+
+       /* Since group has only one device */
+       list_for_each_entry(grp_dev, &group->devices, list)
+               dev = grp_dev->dev;
+
+       /*
+        * Don't hold the group mutex because taking group mutex first and then
+        * the device lock could potentially cause a deadlock as below. Assume
+        * two threads T1 and T2. T1 is trying to change default domain of an
+        * iommu group and T2 is trying to hot unplug a device or release [1] VF
+        * of a PCIe device which is in the same iommu group. T1 takes group
+        * mutex and before it could take device lock assume T2 has taken device
+        * lock and is yet to take group mutex. Now, both the threads will be
+        * waiting for the other thread to release lock. Below, lock order was
+        * suggested.
+        * device_lock(dev);
+        *      mutex_lock(&group->mutex);
+        *              iommu_change_dev_def_domain();
+        *      mutex_unlock(&group->mutex);
+        * device_unlock(dev);
+        *
+        * [1] Typical device release path
+        * device_lock() from device/driver core code
+        *  -> bus_notifier()
+        *   -> iommu_bus_notifier()
+        *    -> iommu_release_device()
+        *     -> ops->release_device() vendor driver calls back iommu core code
+        *      -> mutex_lock() from iommu core code
+        */
+       mutex_unlock(&group->mutex);
+
+       /* Check if the device in the group still has a driver bound to it */
+       device_lock(dev);
+       if (device_is_bound(dev)) {
+               pr_err_ratelimited("Device is still bound to driver\n");
+               ret = -EBUSY;
+               goto out;
+       }
+
+       ret = iommu_change_dev_def_domain(group, req_type);
+       ret = ret ?: count;
+
+out:
+       device_unlock(dev);
+
+       return ret;
+}
-- 
2.19.1

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