IB/mthca: Avoid changing userspace ABI to handle DMA write barrier attribute

Commit cb9fbc5c ("IB: expand ib_umem_get() prototype") changed the
mthca userspace ABI to provide a way for userspace to indicate which
memory regions need the DMA write barrier attribute.  However, it is
possible to handle this without breaking existing userspace, by having
the mthca kernel driver recognize whether it is talking to old or new
userspace, depending on the size of the register MR structure passed in.

The only potential drawback of this is that is allows old userspace
(which has a bug with DMA ordering on large SGI Altix systems) to
continue to run on new kernels, but the advantage of allowing old
userspace to continue to work on unaffected systems seems to outweigh
this, and we can print a warning to push people to upgrade their
userspace.

Signed-off-by: Roland Dreier <rolandd@cisco.com>

1 files changed, 6 insertions, 4 deletions

diff --git a/drivers/infiniband/hw/mthca/mthca_user.h b/drivers/infiniband/hw/mthca/mthca_user.h
index f8cb3b664d37..e1262c942db8 100644
--- a/drivers/infiniband/hw/mthca/mthca_user.h
+++ b/drivers/infiniband/hw/mthca/mthca_user.h
@@ -41,7 +41,7 @@
  * Increment this value if any changes that break userspace ABI
  * compatibility are made.
  */
-#define MTHCA_UVERBS_ABI_VERSION	2
+#define MTHCA_UVERBS_ABI_VERSION	1
 
 /*
  * Make sure that all structs defined in this file remain laid out so
@@ -62,10 +62,12 @@ struct mthca_alloc_pd_resp {
 };
 
 struct mthca_reg_mr {
+/*
+ * Mark the memory region with a DMA attribute that causes
+ * in-flight DMA to be flushed when the region is written to:
+ */
+#define MTHCA_MR_DMASYNC	0x1
 	__u32 mr_attrs;
-#define MTHCA_MR_DMASYNC 0x1
-/* mark the memory region with a DMA attribute that causes
- * in-flight DMA to be flushed when the region is written to */
 	__u32 reserved;
 };