[openib-general] registering read-only memory

[Michael S. Tsirkin]

Roland, the following code snippet:

const char foo[]="Michael Tsirkin";
ibv_reg_mr(ctx->pd, foo, strlen(foo), 0);

exposes two problems with ibv_reg_mr:

1. Compiling this code I get a warning:
warning: passing arg 2 of `ibv_reg_mr' discards qualifiers
from pointer target type.

Same if foo is declared volatile.

I suggest changing ibv_reg_mr to accept const volatile void *
as a second parameter.
const is OK since ibv_reg_mr never actually writes to the buffer.
volatile is needed by some applications to prevent the compiler from
assuming it can re-order accesses to this buffer.

Patch attached (below).

2. ibv_reg_mr fails.
Why is that?


System details:

>gcc --version
gcc (GCC) 3.3.3 (SuSE Linux)

>uname -a
Linux swlab156 2.6.11-openib #29 SMP Mon Apr 18 16:17:51 IDT 2005
x86_64 x86_64 x86_64 GNU/Linux

Make ibv_reg_mr accept buffer as volatile const *.

Signed-off-by: Michael S. Tsirkin <mst at mellanox.co.il>

Index: libibverbs/include/infiniband/verbs.h
===================================================================
--- libibverbs/include/infiniband/verbs.h	(revision 2408)
+++ libibverbs/include/infiniband/verbs.h	(working copy)
@@ -499,7 +499,7 @@ extern int ibv_dealloc_pd(struct ibv_pd 
 /**
  * ibv_reg_mr - Register a memory region
  */
-extern struct ibv_mr *ibv_reg_mr(struct ibv_pd *pd, void *addr,
+extern struct ibv_mr *ibv_reg_mr(struct ibv_pd *pd, volatile const void *addr,
 				 size_t length, enum ibv_access_flags access);
 
 /**
Index: libibverbs/src/verbs.c
===================================================================
--- libibverbs/src/verbs.c	(revision 2408)
+++ libibverbs/src/verbs.c	(working copy)
@@ -64,7 +64,7 @@ int ibv_dealloc_pd(struct ibv_pd *pd)
 	return pd->context->ops.dealloc_pd(pd);
 }
 
-struct ibv_mr *ibv_reg_mr(struct ibv_pd *pd, void *addr,
+struct ibv_mr *ibv_reg_mr(struct ibv_pd *pd, volatile const void *addr,
 			  size_t length, enum ibv_access_flags access)
 {
 	struct ibv_mr *mr;
Index: libmthca/src/mthca.h
===================================================================
--- libmthca/src/mthca.h	(revision 2408)
+++ libmthca/src/mthca.h	(working copy)
@@ -260,7 +260,7 @@ extern int mthca_query_port(struct ibv_c
 extern struct ibv_pd *mthca_alloc_pd(struct ibv_context *context);
 extern int mthca_free_pd(struct ibv_pd *pd);
 
-extern struct ibv_mr *mthca_reg_mr(struct ibv_pd *pd, void *addr,
+extern struct ibv_mr *mthca_reg_mr(struct ibv_pd *pd, volatile const void *addr,
 				   size_t length, enum ibv_access_flags access);
 extern int mthca_dereg_mr(struct ibv_mr *mr);
 
Index: libmthca/src/verbs.c
===================================================================
--- libmthca/src/verbs.c	(revision 2408)
+++ libmthca/src/verbs.c	(working copy)
@@ -93,7 +93,8 @@ int mthca_free_pd(struct ibv_pd *pd)
 	return 0;
 }
 
-static struct ibv_mr *__mthca_reg_mr(struct ibv_pd *pd, void *addr,
+static struct ibv_mr *__mthca_reg_mr(struct ibv_pd *pd,
+				     volatile const void *addr,
 				     size_t length, uint64_t hca_va,
 				     enum ibv_access_flags access)
 {
@@ -113,7 +114,7 @@ static struct ibv_mr *__mthca_reg_mr(str
 	return mr;
 }
 
-struct ibv_mr *mthca_reg_mr(struct ibv_pd *pd, void *addr,
+struct ibv_mr *mthca_reg_mr(struct ibv_pd *pd, volatile const void *addr,
 			    size_t length, enum ibv_access_flags access)
 {
 	return __mthca_reg_mr(pd, addr, length, (uintptr_t) addr, access);
-- 
MST - Michael S. Tsirkin