[ofa-general] page mask calculation in mthca mthca_reg_phys_mr

[Roland Dreier]

 > I think there is a similar bug in mthca as in cxgb3
 > (http://lists.openfabrics.org/pipermail/general/2008-January/045246.html),

 > It can get wrong mask as start address of the first fragment and end
 > address of the last fragment are ignored, here is some example of this:

We had a fairly long discussion about this just now.  It turns out
there is a bug, but not the one you are worried about.  It's fine to
ignore the start address of the first fragment and the end address of
the last fragment, since the starting virtual address and length of
the region will take care of that.  The problem is if the virtual
address has the wrong alignment.

i am planning on merging the patch below, which simplifies things and
also fixes the bug:

diff --git a/drivers/infiniband/hw/mthca/mthca_provider.c b/drivers/infiniband/hw/mthca/mthca_provider.c
index 6bcde1c..19b7f61 100644
--- a/drivers/infiniband/hw/mthca/mthca_provider.c
+++ b/drivers/infiniband/hw/mthca/mthca_provider.c
@@ -923,17 +923,13 @@ static struct ib_mr *mthca_reg_phys_mr(struct ib_pd       *pd,
 	struct mthca_mr *mr;
 	u64 *page_list;
 	u64 total_size;
-	u64 mask;
+	unsigned long mask;
 	int shift;
 	int npages;
 	int err;
 	int i, j, n;
 
-	/* First check that we have enough alignment */
-	if ((*iova_start & ~PAGE_MASK) != (buffer_list[0].addr & ~PAGE_MASK))
-		return ERR_PTR(-EINVAL);
-
-	mask = 0;
+	mask = buffer_list[0].addr ^ *iova_start;
 	total_size = 0;
 	for (i = 0; i < num_phys_buf; ++i) {
 		if (i != 0)
@@ -947,17 +943,7 @@ static struct ib_mr *mthca_reg_phys_mr(struct ib_pd       *pd,
 	if (mask & ~PAGE_MASK)
 		return ERR_PTR(-EINVAL);
 
-	/* Find largest page shift we can use to cover buffers */
-	for (shift = PAGE_SHIFT; shift < 31; ++shift)
-		if (num_phys_buf > 1) {
-			if ((1ULL << shift) & mask)
-				break;
-		} else {
-			if (1ULL << shift >=
-			    buffer_list[0].size +
-			    (buffer_list[0].addr & ((1ULL << shift) - 1)))
-				break;
-		}
+	shift = __ffs(mask | 1 << 31);
 
 	buffer_list[0].size += buffer_list[0].addr & ((1ULL << shift) - 1);
 	buffer_list[0].addr &= ~0ull << shift;