[RFC PATCH 1/3] iommu: Move IOMMU pagesize check to attach_device

[Sven Peter <sven@svenpeter.dev>]

The iova allocator is capable of handling any granularity which is a power
of two. Remove the much stronger condition that the granularity must be
smaller or equal to the CPU page size from a BUG_ON there.
Instead, check this condition during __iommu_attach_device and fail
gracefully.

Signed-off-by: Sven Peter <sven@svenpeter.dev>
---
 drivers/iommu/iommu.c | 34 +++++++++++++++++++++++++++++++---
 drivers/iommu/iova.c  |  7 ++++---
 2 files changed, 35 insertions(+), 6 deletions(-)

diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index 1de503ddb343..5854a4ef5681 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -78,6 +78,8 @@ static struct iommu_domain *__iommu_domain_alloc(struct bus_type *bus,
 						 unsigned type);
 static int __iommu_attach_device(struct iommu_domain *domain,
 				 struct device *dev);
+static void __iommu_detach_device(struct iommu_domain *domain,
+				  struct device *dev);
 static int __iommu_attach_group(struct iommu_domain *domain,
 				struct iommu_group *group);
 static void __iommu_detach_group(struct iommu_domain *domain,
@@ -1964,6 +1966,18 @@ void iommu_domain_free(struct iommu_domain *domain)
 }
 EXPORT_SYMBOL_GPL(iommu_domain_free);
 
+static int iommu_check_page_size(struct iommu_domain *domain)
+{
+	if (!(domain->type & __IOMMU_DOMAIN_PAGING))
+		return 0;
+
+	if ((1 << __ffs(domain->pgsize_bitmap)) > PAGE_SIZE) {
+		pr_warn("IOMMU page size cannot represent CPU pages.\n");
+		return -EFAULT;
+	}
+
+	return 0;
+}
 static int __iommu_attach_device(struct iommu_domain *domain,
 				 struct device *dev)
 {
@@ -1973,9 +1987,23 @@ static int __iommu_attach_device(struct iommu_domain *domain,
 		return -ENODEV;
 
 	ret = domain->ops->attach_dev(domain, dev);
-	if (!ret)
-		trace_attach_device_to_domain(dev);
-	return ret;
+	if (ret)
+		return ret;
+
+	/*
+	 * Check that CPU pages can be represented by the IOVA granularity.
+	 * This has to be done after ops->attach_dev since many IOMMU drivers
+	 * only limit domain->pgsize_bitmap after having attached the first
+	 * device.
+	 */
+	ret = iommu_check_page_size(domain);
+	if (ret) {
+		__iommu_detach_device(domain, dev);
+		return ret;
+	}
+
+	trace_attach_device_to_domain(dev);
+	return 0;
 }
 
 int iommu_attach_device(struct iommu_domain *domain, struct device *dev)
diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
index b6cf5f16123b..e0f8adde0f1b 100644
--- a/drivers/iommu/iova.c
+++ b/drivers/iommu/iova.c
@@ -50,10 +50,11 @@ init_iova_domain(struct iova_domain *iovad, unsigned long granule,
 {
 	/*
 	 * IOVA granularity will normally be equal to the smallest
-	 * supported IOMMU page size; both *must* be capable of
-	 * representing individual CPU pages exactly.
+	 * supported IOMMU page size; while both usually are capable of
+	 * representing individual CPU pages exactly the IOVA allocator
+	 * supports any granularities that are an exact power of two.
 	 */
-	BUG_ON((granule > PAGE_SIZE) || !is_power_of_2(granule));
+	BUG_ON(!is_power_of_2(granule));
 
 	spin_lock_init(&iovad->iova_rbtree_lock);
 	iovad->rbroot = RB_ROOT;
-- 
2.25.1