iommu/sva: Restore SVA handle sharing

Prior to commit 092edadd ("iommu: Support mm PASID 1:n with sva
domains") the code allowed a SVA handle to be bound multiple times to the
same (mm, device) pair. This was alluded to in the kdoc comment, but we
had understood this to be more a remark about allowing multiple devices,
not a literal same-driver re-opening the same SVA.

It turns out uacce and idxd were both relying on the core code to handle
reference counting for same-device same-mm scenarios. As this looks hard
to resolve in the drivers bring it back to the core code.

The new design has changed the meaning of the domain->users refcount to
refer to the number of devices that are sharing that domain for the same
mm. This is part of the design to lift the SVA domain de-duplication out
of the drivers.

Return the old behavior by explicitly de-duplicating the struct iommu_sva
handle. The same (mm, device) will return the same handle pointer and the
core code will handle tracking this. The last unbind of the handle will
destroy it.

Fixes: 092edadd ("iommu: Support mm PASID 1:n with sva domains")
Reported-by: Zhangfei Gao <zhangfei.gao@linaro.org>
Closes: https://lore.kernel.org/all/20240221110658.529-1-zhangfei.gao@linaro.org/Tested-by: Zhangfei Gao <zhangfei.gao@linaro.org>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/0-v1-9455fc497a6f+3b4-iommu_sva_sharing_jgg@nvidia.comSigned-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/iommu-sva.c

@@ -41,6 +41,7 @@ static struct iommu_mm_data *iommu_alloc_mm_data(struct mm_struct *mm, struct de
 	}
 	iommu_mm->pasid = pasid;
 	INIT_LIST_HEAD(&iommu_mm->sva_domains);
+	INIT_LIST_HEAD(&iommu_mm->sva_handles);
 	/*
 	 * Make sure the write to mm->iommu_mm is not reordered in front of
 	 * initialization to iommu_mm fields. If it does, readers may see a
@@ -82,6 +83,14 @@ struct iommu_sva *iommu_sva_bind_device(struct device *dev, struct mm_struct *mm
 		goto out_unlock;
 	}
 
+	list_for_each_entry(handle, &mm->iommu_mm->sva_handles, handle_item) {
+		if (handle->dev == dev) {
+			refcount_inc(&handle->users);
+			mutex_unlock(&iommu_sva_lock);
+			return handle;
+		}
+	}
+
 	handle = kzalloc(sizeof(*handle), GFP_KERNEL);
 	if (!handle) {
 		ret = -ENOMEM;
@@ -108,7 +117,9 @@ struct iommu_sva *iommu_sva_bind_device(struct device *dev, struct mm_struct *mm
 	if (ret)
 		goto out_free_domain;
 	domain->users = 1;
+	refcount_set(&handle->users, 1);
 	list_add(&domain->next, &mm->iommu_mm->sva_domains);
+	list_add(&handle->handle_item, &mm->iommu_mm->sva_handles);
 
 out:
 	mutex_unlock(&iommu_sva_lock);
@@ -141,6 +152,12 @@ void iommu_sva_unbind_device(struct iommu_sva *handle)
 	struct device *dev = handle->dev;
 
 	mutex_lock(&iommu_sva_lock);
+	if (!refcount_dec_and_test(&handle->users)) {
+		mutex_unlock(&iommu_sva_lock);
+		return;
+	}
+	list_del(&handle->handle_item);
+
 	iommu_detach_device_pasid(domain, dev, iommu_mm->pasid);
 	if (--domain->users == 0) {
 		list_del(&domain->next);

include/linux/iommu.h

@@ -892,11 +892,14 @@ struct iommu_fwspec {
 struct iommu_sva {
 	struct device			*dev;
 	struct iommu_domain		*domain;
+	struct list_head		handle_item;
+	refcount_t			users;
 };
 
 struct iommu_mm_data {
 	u32			pasid;
 	struct list_head	sva_domains;
+	struct list_head	sva_handles;
 };
 
 int iommu_fwspec_init(struct device *dev, struct fwnode_handle *iommu_fwnode,