iommu: Use bitmap to calculate page size in iommu_pgsize()

Avoid the potential for shifting values by amounts greater than the
width of their type by using a bitmap to compute page size in
iommu_pgsize().

Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>
Signed-off-by: Georgi Djakov <quic_c_gdjako@quicinc.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/1623850736-389584-6-git-send-email-quic_c_gdjako@quicinc.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>

Author: willdeacon

drivers/iommu/iommu.c

@@ -8,6 +8,7 @@
 
 #include <linux/device.h>
 #include <linux/kernel.h>
+#include <linux/bits.h>
 #include <linux/bug.h>
 #include <linux/types.h>
 #include <linux/init.h>
@@ -2378,30 +2379,22 @@ static size_t iommu_pgsize(struct iommu_domain *domain,
 			   unsigned long addr_merge, size_t size)
 {
 	unsigned int pgsize_idx;
+	unsigned long pgsizes;
 	size_t pgsize;
 
-	/* Max page size that still fits into 'size' */
-	pgsize_idx = __fls(size);
+	/* Page sizes supported by the hardware and small enough for @size */
+	pgsizes = domain->pgsize_bitmap & GENMASK(__fls(size), 0);
 
-	/* need to consider alignment requirements ? */
-	if (likely(addr_merge)) {
-		/* Max page size allowed by address */
-		unsigned int align_pgsize_idx = __ffs(addr_merge);
-		pgsize_idx = min(pgsize_idx, align_pgsize_idx);
-	}
-
-	/* build a mask of acceptable page sizes */
-	pgsize = (1UL << (pgsize_idx + 1)) - 1;
-
-	/* throw away page sizes not supported by the hardware */
-	pgsize &= domain->pgsize_bitmap;
+	/* Constrain the page sizes further based on the maximum alignment */
+	if (likely(addr_merge))
+		pgsizes &= GENMASK(__ffs(addr_merge), 0);
 
-	/* make sure we're still sane */
-	BUG_ON(!pgsize);
+	/* Make sure we have at least one suitable page size */
+	BUG_ON(!pgsizes);
 
-	/* pick the biggest page */
-	pgsize_idx = __fls(pgsize);
-	pgsize = 1UL << pgsize_idx;
+	/* Pick the biggest page size remaining */
+	pgsize_idx = __fls(pgsizes);
+	pgsize = BIT(pgsize_idx);
 
 	return pgsize;
 }