iommu/arm-smmu-v3-sva: Remove bond refcount

Always allocate a new arm_smmu_bond in __arm_smmu_sva_bind and remove
the bond refcount since arm_smmu_bond can never be shared across calls
to __arm_smmu_sva_bind.

The iommu framework will not allocate multiple SVA domains for the same
(device/mm) pair, nor will it call set_dev_pasid for a device if a
domain is already attached on the given pasid. There's also a one-to-one
mapping between MM and PASID. __arm_smmu_sva_bind is therefore never
called with the same (device/mm) pair, and so there's no reason to try
and normalize allocations of the arm_smmu_bond struct for a (device/mm)
pair across set_dev_pasid.

Signed-off-by: Michael Shavit <mshavit@google.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20230905194849.v1.2.Id3ab7cf665bcead097654937233a645722a4cce3@changeid
Signed-off-by: Will Deacon <will@kernel.org>

Author: shavitmichael

drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-sva.c

@@ -28,7 +28,6 @@ struct arm_smmu_bond {
 	struct mm_struct		*mm;
 	struct arm_smmu_mmu_notifier	*smmu_mn;
 	struct list_head		list;
-	refcount_t			refs;
 };
 
 #define sva_to_bond(handle) \
@@ -386,20 +385,11 @@ static int __arm_smmu_sva_bind(struct device *dev, struct mm_struct *mm)
 	if (!master || !master->sva_enabled)
 		return -ENODEV;
 
-	/* If bind() was already called for this {dev, mm} pair, reuse it. */
-	list_for_each_entry(bond, &master->bonds, list) {
-		if (bond->mm == mm) {
-			refcount_inc(&bond->refs);
-			return &bond->sva;
-		}
-	}
-
 	bond = kzalloc(sizeof(*bond), GFP_KERNEL);
 	if (!bond)
 		return -ENOMEM;
 
 	bond->mm = mm;
-	refcount_set(&bond->refs, 1);
 
 	bond->smmu_mn = arm_smmu_mmu_notifier_get(smmu_domain, mm);
 	if (IS_ERR(bond->smmu_mn)) {
@@ -578,7 +568,7 @@ void arm_smmu_sva_remove_dev_pasid(struct iommu_domain *domain,
 		}
 	}
 
-	if (!WARN_ON(!bond) && refcount_dec_and_test(&bond->refs)) {
+	if (!WARN_ON(!bond)) {
 		list_del(&bond->list);
 		arm_smmu_mmu_notifier_put(bond->smmu_mn);
 		kfree(bond);