iommu/iova: Avoid over-allocating when size-aligned

Currently, allocating a size-aligned IOVA region quietly adjusts the
actual allocation size in the process, returning a rounded-up
power-of-two-sized allocation. This results in mismatched behaviour in
the IOMMU driver if the original size was not a power of two, where the
original size is mapped, but the rounded-up IOVA size is unmapped.

Whilst some IOMMUs will happily unmap already-unmapped pages, others
consider this an error, so fix it by computing the necessary alignment
padding without altering the actual allocation size. Also clean up by
making pad_size unsigned, since its callers always pass unsigned values
and negative padding makes little sense here anyway.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

2 files changed, 8 insertions, 17 deletions

diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c
index a98a7b27aca1..92101597cede 100644
--- a/drivers/iommu/intel-iommu.c
+++ b/drivers/iommu/intel-iommu.c
@@ -3233,6 +3233,8 @@ static struct iova *intel_alloc_iova(struct device *dev,
 
         /* Restrict dma_mask to the width that the iommu can handle */
         dma_mask = min_t(uint64_t, DOMAIN_MAX_ADDR(domain->gaw), dma_mask);
+        /* Ensure we reserve the whole size-aligned region */
+        nrpages = __roundup_pow_of_two(nrpages);
 
         if (!dmar_forcedac && dma_mask > DMA_BIT_MASK(32)) {
                 /*
diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
index b7c3d923f3e1..29f2efcf668e 100644
--- a/drivers/iommu/iova.c
+++ b/drivers/iommu/iova.c
@@ -120,19 +120,14 @@ __cached_rbnode_delete_update(struct iova_domain *iovad, struct iova *free)
         }
 }
 
-/* Computes the padding size required, to make the
- * the start address naturally aligned on its size
+/*
+ * Computes the padding size required, to make the start address
+ * naturally aligned on the power-of-two order of its size
  */
-static int
-iova_get_pad_size(int size, unsigned int limit_pfn)
+static unsigned int
+iova_get_pad_size(unsigned int size, unsigned int limit_pfn)
 {
-        unsigned int pad_size = 0;
-        unsigned int order = ilog2(size);
-
-        if (order)
-                pad_size = (limit_pfn + 1) % (1 << order);
-
-        return pad_size;
+        return (limit_pfn + 1 - size) & (__roundup_pow_of_two(size) - 1);
 }
 
 static int __alloc_and_insert_iova_range(struct iova_domain *iovad,
@@ -265,12 +260,6 @@ alloc_iova(struct iova_domain *iovad, unsigned long size,
         if (!new_iova)
                 return NULL;
 
-        /* If size aligned is set then round the size to
-         * to next power of two.
-         */
-        if (size_aligned)
-                size = __roundup_pow_of_two(size);
-
         ret = __alloc_and_insert_iova_range(iovad, size, limit_pfn,
                         new_iova, size_aligned);