Revert "FIB: optimise for src memory allocations"

This reverts commit 84517cfd1508f6da24937f310f7fffe752f22584.

Change-Id: Ic7eeffa2ed4607d3d653f34b93c20c833c789ee1
Signed-off-by: Neale Ranns <nranns@cisco.com>

5 files changed, 121 insertions, 264 deletions

diff --git a/src/vnet/fib/fib_entry.c b/src/vnet/fib/fib_entry.c
index a9ca0f20f6d..35716cacc29 100644
--- a/src/vnet/fib/fib_entry.c
+++ b/src/vnet/fib/fib_entry.c
@@ -213,33 +213,29 @@ fib_entry_last_lock_gone (fib_node_t *node)
     FIB_ENTRY_DBG(fib_entry, "last-lock");
 
     fib_node_deinit(&fib_entry->fe_node);
+    // FIXME -RR Backwalk
 
     ASSERT(0 == vec_len(fib_entry->fe_delegates));
     vec_free(fib_entry->fe_delegates);
-    if (fib_entry_has_multiple_srcs(fib_entry))
-    {
-        vec_free(fib_entry->fe_u_src.fe_srcs);
-    }
+    vec_free(fib_entry->fe_srcs);
     pool_put(fib_entry_pool, fib_entry);
 }
 
-static const fib_entry_src_t*
+static fib_entry_src_t*
 fib_entry_get_best_src_i (const fib_entry_t *fib_entry)
 {
-    const fib_entry_src_t *bsrc;
+    fib_entry_src_t *bsrc;
 
     /*
      * the enum of sources is deliberately arranged in priority order
      */
-    if (fib_entry_has_multiple_srcs(fib_entry))
+    if (0 == vec_len(fib_entry->fe_srcs))
     {
-        ASSERT(vec_len(fib_entry->fe_u_src.fe_srcs));
-
-        bsrc = vec_elt_at_index(fib_entry->fe_u_src.fe_srcs, 0);
+	bsrc = NULL;
     }
     else
     {
-        bsrc = &fib_entry->fe_u_src.fe_src;
+	bsrc = vec_elt_at_index(fib_entry->fe_srcs, 0);
     }
 
     return (bsrc);
@@ -252,7 +248,7 @@ fib_entry_src_get_source (const fib_entry_src_t *esrc)
     {
 	return (esrc->fes_src);
     }
-    return (FIB_SOURCE_INVALID);
+    return (FIB_SOURCE_MAX);
 }
 
 static fib_entry_flag_t
@@ -266,18 +262,6 @@ fib_entry_src_get_flags (const fib_entry_src_t *esrc)
 }
 
 fib_entry_flag_t
-fib_entry_get_flags_i (const fib_entry_t *fib_entry)
-{
-    const fib_entry_src_t *esrc;
-
-    esrc = fib_entry_get_best_src_i(fib_entry);
-
-    ASSERT(esrc);
-
-    return (esrc->fes_entry_flags);
-}
-
-fib_entry_flag_t
 fib_entry_get_flags (fib_node_index_t fib_entry_index)
 {
     return (fib_entry_get_flags_i(fib_entry_get(fib_entry_index)));
@@ -346,18 +330,11 @@ fib_entry_show_memory (void)
 
     pool_foreach(entry, fib_entry_pool,
     ({
-        if (fib_entry_has_multiple_srcs(entry))
-        {
-            n_srcs += vec_len(entry->fe_u_src.fe_srcs);
-            vec_foreach(esrc, entry->fe_u_src.fe_srcs)
-            {
-                n_exts += fib_path_ext_list_length(&esrc->fes_path_exts);
-            }
-        }
-        else
-        {
-            n_exts += fib_path_ext_list_length(&entry->fe_u_src.fe_src.fes_path_exts);
-        }
+	n_srcs += vec_len(entry->fe_srcs);
+	vec_foreach(esrc, entry->fe_srcs)
+	{
+	    n_exts += fib_path_ext_list_length(&esrc->fes_path_exts);
+	}
     }));
 
     fib_show_memory_usage("Entry Source",
@@ -818,10 +795,10 @@ fib_entry_special_add (fib_node_index_t fib_entry_index,
 		       fib_entry_flag_t flags,
 		       const dpo_id_t *dpo)
 {
-    const fib_entry_src_t *bsrc;
     fib_source_t best_source;
     fib_entry_flag_t bflags;
     fib_entry_t *fib_entry;
+    fib_entry_src_t *bsrc;
 
     fib_entry = fib_entry_get(fib_entry_index);
 
@@ -839,10 +816,10 @@ fib_entry_special_update (fib_node_index_t fib_entry_index,
 			  fib_entry_flag_t flags,
 			  const dpo_id_t *dpo)
 {
-    const fib_entry_src_t *bsrc;
     fib_source_t best_source;
     fib_entry_flag_t bflags;
     fib_entry_t *fib_entry;
+    fib_entry_src_t *bsrc;
 
     fib_entry = fib_entry_get(fib_entry_index);
 
@@ -861,10 +838,10 @@ fib_entry_path_add (fib_node_index_t fib_entry_index,
 		    fib_entry_flag_t flags,
 		    const fib_route_path_t *rpath)
 {
-    const fib_entry_src_t *bsrc;
     fib_source_t best_source;
     fib_entry_flag_t bflags;
     fib_entry_t *fib_entry;
+    fib_entry_src_t *bsrc;
 
     ASSERT(1 == vec_len(rpath));
 
@@ -923,11 +900,11 @@ fib_entry_path_remove (fib_node_index_t fib_entry_index,
 		       fib_source_t source,
 		       const fib_route_path_t *rpath)
 {
-    const fib_entry_src_t *bsrc;
     fib_entry_src_flag_t sflag;
     fib_source_t best_source;
     fib_entry_flag_t bflags;
     fib_entry_t *fib_entry;
+    fib_entry_src_t *bsrc;
 
     ASSERT(1 == vec_len(rpath));
 
@@ -974,7 +951,7 @@ fib_entry_path_remove (fib_node_index_t fib_entry_index,
 	    bsrc = fib_entry_get_best_src_i(fib_entry);
 	    best_source = fib_entry_src_get_source(bsrc);
 
-	    if (FIB_SOURCE_INVALID == best_source) {
+	    if (FIB_SOURCE_MAX == best_source) {
 		/*
 		 * no more sources left. this entry is toast.
 		 */
@@ -1019,11 +996,11 @@ fib_entry_src_flag_t
 fib_entry_special_remove (fib_node_index_t fib_entry_index,
 			  fib_source_t source)
 {
-    const fib_entry_src_t *bsrc;
     fib_entry_src_flag_t sflag;
     fib_source_t best_source;
     fib_entry_flag_t bflags;
     fib_entry_t *fib_entry;
+    fib_entry_src_t *bsrc;
 
     fib_entry = fib_entry_get(fib_entry_index);
     ASSERT(NULL != fib_entry);
@@ -1064,7 +1041,7 @@ fib_entry_special_remove (fib_node_index_t fib_entry_index,
 	    bsrc = fib_entry_get_best_src_i(fib_entry);
 	    best_source = fib_entry_src_get_source(bsrc);
 
-	    if (FIB_SOURCE_INVALID == best_source) {
+	    if (FIB_SOURCE_MAX == best_source) {
 		/*
 		 * no more sources left. this entry is toast.
 		 */
@@ -1121,10 +1098,10 @@ fib_entry_update (fib_node_index_t fib_entry_index,
 		  fib_entry_flag_t flags,
 		  const fib_route_path_t *paths)
 {
-    const fib_entry_src_t *bsrc;
     fib_source_t best_source;
     fib_entry_flag_t bflags;
     fib_entry_t *fib_entry;
+    fib_entry_src_t *bsrc;
 
     fib_entry = fib_entry_get(fib_entry_index);
     ASSERT(NULL != fib_entry);
@@ -1384,8 +1361,8 @@ fib_entry_get_resolving_interface (fib_node_index_t entry_index)
 fib_source_t
 fib_entry_get_best_source (fib_node_index_t entry_index)
 {
-    const fib_entry_src_t *bsrc;
     fib_entry_t *fib_entry;
+    fib_entry_src_t *bsrc;
 
     fib_entry = fib_entry_get(entry_index);
 
diff --git a/src/vnet/fib/fib_entry.h b/src/vnet/fib/fib_entry.h
index cd954e3a15c..cd2a685b765 100644
--- a/src/vnet/fib/fib_entry.h
+++ b/src/vnet/fib/fib_entry.h
@@ -29,10 +29,6 @@
  */
 typedef enum fib_source_t_ {
     /**
-     * An invalid source of value 0
-     */
-    FIB_SOURCE_INVALID,
-    /**
      * Marker. Add new values after this one.
      */
     FIB_SOURCE_FIRST,
@@ -146,7 +142,6 @@ STATIC_ASSERT (sizeof(fib_source_t) == 1,
 #define FIB_SOURCE_MAX (FIB_SOURCE_LAST+1)
 
 #define FIB_SOURCES {					\
-    [FIB_SOURCE_INVALID] = "invalid",			\
     [FIB_SOURCE_SPECIAL] = "special",			\
     [FIB_SOURCE_INTERFACE] = "interface",		\
     [FIB_SOURCE_PROXY] = "proxy",                       \
@@ -382,29 +377,6 @@ typedef struct fib_entry_src_t_ {
 } fib_entry_src_t;
 
 /**
- * FIB entry flags.
- *  these are stored in the pad space within the fib_node_t
- */
-typedef enum fib_entry_node_attribute_t_
-{
-    /**
-     * FIB entry has multiple sources, so the fe_srcs union
-     * uses the vector
-     */
-    FIB_ENTRY_NODE_ATTR_MULTIPLE_SRCS,
-} fib_entry_node_attribute_t;
-
-#define FIB_ENTRY_NODE_FLAG_NAMES {                            \
-    [FIB_ENTRY_NODE_ATTR_MULTIPLE_SRCS] = "multiple-srcs",     \
-}
-
-typedef enum fib_entry_node_flags_t_
-{
-    FIB_ENTRY_NODE_FLAG_MULTIPLE_SRCS = (1 << FIB_ENTRY_NODE_ATTR_MULTIPLE_SRCS),
-} fib_entry_node_flags_t;
-
-
-/**
  * An entry in a FIB table.
  *
  * This entry represents a route added to the FIB that is stored
@@ -437,20 +409,12 @@ typedef struct fib_entry_t_ {
      *     type to derive the EOS bit value.
      */
     dpo_id_t fe_lb;
-
     /**
-     * Source info.
-     * in the majority of cases a FIB entry will have only one source.
-     * so to save the extra memory allocation of the source's vector, we
-     * store space for one source inline. When two sources are present, 
-     * we burn extra memory.
-     * The union is switched based on the FIB_ENTRY_NODE_FLAG_MULTIPLE_SRCS
+     * Vector of source infos.
+     * Most entries will only have 1 source. So we optimise for memory usage,
+     * which is preferable since we have many entries.
      */
-    union {
-        fib_entry_src_t *fe_srcs;
-        fib_entry_src_t fe_src;
-    } fe_u_src;
-
+    fib_entry_src_t *fe_srcs;
     /**
      * the path-list for which this entry is a child. This is also the path-list
      * that is contributing forwarding for this entry.
diff --git a/src/vnet/fib/fib_entry_src.c b/src/vnet/fib/fib_entry_src.c
index acb8579e85e..214dafe9b8d 100644
--- a/src/vnet/fib/fib_entry_src.c
+++ b/src/vnet/fib/fib_entry_src.c
@@ -61,62 +61,13 @@ fib_entry_src_action_init (fib_entry_t *fib_entry,
 	fib_entry_src_vft[source].fesv_init(&esrc);
     }
 
-    if (fib_entry_has_multiple_srcs(fib_entry))
-    {
-        vec_add1(fib_entry->fe_u_src.fe_srcs, esrc);
-
-        vec_sort_with_function(fib_entry->fe_u_src.fe_srcs,
-                               fib_entry_src_cmp_for_sort);
-    }
-    else
-    {
-        /*
-         * is this the very first source
-         */
-        if (FIB_SOURCE_INVALID == fib_entry->fe_u_src.fe_src.fes_src)
-        {
-            clib_memcpy(&fib_entry->fe_u_src.fe_src, &esrc, sizeof(esrc));
-        }
-        else
-        {
-            /*
-             * transitioning to multiple sources.
-             * allocate the vecotr of sources.
-             */
-            fib_entry_src_t *srcs = NULL;
-
-            vec_validate(srcs, 1);
-
-            /*
-             * sorted insert
-             */
-            if (fib_entry->fe_u_src.fe_src.fes_src < esrc.fes_src)
-            {
-                srcs[0] = fib_entry->fe_u_src.fe_src;
-                srcs[1] = esrc;
-            }
-            else
-            {
-                srcs[0] = esrc;
-                srcs[1] = fib_entry->fe_u_src.fe_src;
-            }
-            memset(&fib_entry->fe_u_src.fe_src, 0,
-                   sizeof(fib_entry->fe_u_src.fe_src));
-            fib_entry->fe_u_src.fe_srcs = srcs;
-
-            fib_entry->fe_node.fn_pad |= FIB_ENTRY_NODE_FLAG_MULTIPLE_SRCS;
-        }
-    }
-}
-
-u32
-fib_entry_has_multiple_srcs(const fib_entry_t * fib_entry)
-{
-    return (fib_entry->fe_node.fn_pad & FIB_ENTRY_NODE_FLAG_MULTIPLE_SRCS);
+    vec_add1(fib_entry->fe_srcs, esrc);
+    vec_sort_with_function(fib_entry->fe_srcs,
+			   fib_entry_src_cmp_for_sort);
 }
 
 static fib_entry_src_t *
-fib_entry_src_find (fib_entry_t *fib_entry,
+fib_entry_src_find (const fib_entry_t *fib_entry,
 		    fib_source_t source,
 		    u32 *index)
 
@@ -125,80 +76,20 @@ fib_entry_src_find (fib_entry_t *fib_entry,
     int ii;
 
     ii = 0;
-    if (fib_entry_has_multiple_srcs(fib_entry))
-    {
-        vec_foreach(esrc, fib_entry->fe_u_src.fe_srcs)
-        {
-            if (esrc->fes_src == source)
-            {
-                if (NULL != index)
-                {
-                    *index = ii;
-                }
-                return (esrc);
-            }
-            else
-            {
-                ii++;
-            }
-        }
-    }
-    else
-    {
-        esrc = &fib_entry->fe_u_src.fe_src;
-        if (esrc->fes_src == source)
-        {
-            if (NULL != index)
-            {
-                *index = -1;
-            }
-            return (esrc);
-        }
-    }
-
-    return (NULL);
-}
-
-static fib_entry_src_t *
-fib_entry_src_delete (fib_entry_t *fib_entry,
-                      u32 index)
-
-{
-    if (-1 == index)
-    {
-        ASSERT(!fib_entry_has_multiple_srcs(fib_entry));
-        memset(&fib_entry->fe_u_src.fe_src, 0,
-               sizeof(fib_entry->fe_u_src.fe_src));
-    }
-    else
+    vec_foreach(esrc, fib_entry->fe_srcs)
     {
-        vec_del1(fib_entry->fe_u_src.fe_srcs, index);
-
-        ASSERT(vec_len(fib_entry->fe_u_src.fe_srcs));
-        if (1 == vec_len(fib_entry->fe_u_src.fe_srcs))
-        {
-            /*
-             * Is there much point in transitioning back?
-             * We've paid the cost of the malloc for the vector,
-             * why not keep it.
-             * Favour memory use. If the expectation that multiple sources
-             * is rare is correct, then we should expect this entry is
-             * unlikely to need the vector again
-             */
-            fib_entry_src_t *srcs;
-
-            srcs = fib_entry->fe_u_src.fe_srcs;
-            fib_entry->fe_node.fn_pad &= ~FIB_ENTRY_NODE_FLAG_MULTIPLE_SRCS;
-            clib_memcpy(&fib_entry->fe_u_src.fe_src,
-                        &srcs[0],
-                        sizeof(fib_entry->fe_u_src.fe_src));
-            vec_free(srcs);
-        }
-        else
-        {
-            vec_sort_with_function(fib_entry->fe_u_src.fe_srcs,
-                                   fib_entry_src_cmp_for_sort);
-        }
+	if (esrc->fes_src == source)
+	{
+	    if (NULL != index)
+	    {
+		*index = ii;
+	    }
+	    return (esrc);
+	}
+	else
+	{
+	    ii++;
+	}
     }
 
     return (NULL);
@@ -217,7 +108,8 @@ fib_entry_is_sourced (fib_node_index_t fib_entry_index,
 
 static fib_entry_src_t *
 fib_entry_src_find_or_create (fib_entry_t *fib_entry,
-			      fib_source_t source)
+			      fib_source_t source,
+			      u32 *index)
 {
     fib_entry_src_t *esrc;
 
@@ -249,7 +141,7 @@ fib_entry_src_action_deinit (fib_entry_t *fib_entry,
     }
 
     fib_path_ext_list_flush(&esrc->fes_path_exts);
-    fib_entry_src_delete(fib_entry, index);
+    vec_del1(fib_entry->fe_srcs, index);
 }
 
 fib_entry_src_cover_res_t
@@ -852,6 +744,23 @@ fib_entry_src_action_deactivate (fib_entry_t *fib_entry,
     fib_entry->fe_sibling = FIB_NODE_INDEX_INVALID;
 }
 
+static void
+fib_entry_src_action_fwd_update (const fib_entry_t *fib_entry,
+				 fib_source_t source)
+{
+    fib_entry_src_t *esrc;
+
+    vec_foreach(esrc, fib_entry->fe_srcs)
+    {
+	if (NULL != fib_entry_src_vft[esrc->fes_src].fesv_fwd_update)
+	{
+	    fib_entry_src_vft[esrc->fes_src].fesv_fwd_update(esrc,
+							     fib_entry,
+							     source);
+	}
+    }
+}
+
 void
 fib_entry_src_action_reactivate (fib_entry_t *fib_entry,
 				 fib_source_t source)
@@ -904,10 +813,11 @@ fib_entry_src_action_reactivate (fib_entry_t *fib_entry,
 	fib_path_list_unlock(path_list_index);
     }
     fib_entry_src_action_install(fib_entry, source);
+    fib_entry_src_action_fwd_update(fib_entry, source);
 }
 
 void
-fib_entry_src_action_installed (fib_entry_t *fib_entry,
+fib_entry_src_action_installed (const fib_entry_t *fib_entry,
 				fib_source_t source)
 {
     fib_entry_src_t *esrc;
@@ -919,6 +829,8 @@ fib_entry_src_action_installed (fib_entry_t *fib_entry,
 	fib_entry_src_vft[source].fesv_installed(esrc,
 						 fib_entry);
     }
+
+    fib_entry_src_action_fwd_update(fib_entry, source);
 }
 
 /*
@@ -938,7 +850,7 @@ fib_entry_src_action_add (fib_entry_t *fib_entry,
     fib_node_index_t fib_entry_index;
     fib_entry_src_t *esrc;
 
-    esrc = fib_entry_src_find_or_create(fib_entry, source);
+    esrc = fib_entry_src_find_or_create(fib_entry, source, NULL);
 
     esrc->fes_ref_count++;
 
@@ -997,7 +909,7 @@ fib_entry_src_action_update (fib_entry_t *fib_entry,
     fib_node_index_t fib_entry_index, old_path_list_index;
     fib_entry_src_t *esrc;
 
-    esrc = fib_entry_src_find_or_create(fib_entry, source);
+    esrc = fib_entry_src_find_or_create(fib_entry, source, NULL);
 
     if (NULL == esrc)
 	return (fib_entry_src_action_add(fib_entry, source, flags, dpo));
@@ -1457,6 +1369,29 @@ fib_entry_get_flags_for_source (fib_node_index_t entry_index,
     return (FIB_ENTRY_FLAG_NONE);
 }
 
+fib_entry_flag_t
+fib_entry_get_flags_i (const fib_entry_t *fib_entry)
+{
+    fib_entry_flag_t flags;
+
+    /*
+     * the vector of sources is deliberately arranged in priority order
+     */
+    if (0 == vec_len(fib_entry->fe_srcs))
+    {
+	flags = FIB_ENTRY_FLAG_NONE;
+    }
+    else
+    {
+	fib_entry_src_t *esrc;
+
+	esrc = vec_elt_at_index(fib_entry->fe_srcs, 0);
+	flags = esrc->fes_entry_flags;
+    }
+
+    return (flags);
+}
+
 void
 fib_entry_set_source_data (fib_node_index_t fib_entry_index,
                            fib_source_t source,
diff --git a/src/vnet/fib/fib_entry_src.h b/src/vnet/fib/fib_entry_src.h
index a19fae10a1c..35c43936a1f 100644
--- a/src/vnet/fib/fib_entry_src.h
+++ b/src/vnet/fib/fib_entry_src.h
@@ -105,6 +105,15 @@ typedef fib_entry_src_cover_res_t (*fib_entry_src_cover_update_t)(
     const fib_entry_t *fib_entry);
 
 /**
+ * Forwarding updated. Notification that the forwarding information for the
+ * entry has been updated. This notification is sent to all sources, not just
+ * the active best.
+ */
+typedef void (*fib_entry_src_fwd_update_t)(fib_entry_src_t *src,
+					   const fib_entry_t *fib_entry,
+					   fib_source_t best_source);
+
+/**
  * Installed. Notification that the source is now installed as
  * the entry's forwarding source.
  */
@@ -173,56 +182,22 @@ typedef struct fib_entry_src_vft_t_ {
     fib_entry_src_cover_update_t fesv_cover_update;
     fib_entry_src_format_t fesv_format;
     fib_entry_src_installed_t fesv_installed;
+    fib_entry_src_fwd_update_t fesv_fwd_update;
     fib_entry_src_get_data_t fesv_get_data;
     fib_entry_src_set_data_t fesv_set_data;
 } fib_entry_src_vft_t;
 
-#define FOR_EACH_SRC_ADDED(_entry, _src, _source, action)        \
-{                                                                \
-    if (fib_entry_has_multiple_srcs(_entry))                     \
-    {                                                            \
-        vec_foreach(_src, _entry->fe_u_src.fe_srcs)              \
-        {                                                        \
-            if (_src->fes_flags & FIB_ENTRY_SRC_FLAG_ADDED) {    \
-                _source = _src->fes_src;                         \
-                do {                                             \
-                    action;                                      \
-                } while(0);                                      \
-            }                                                    \
-        }                                                        \
-    }                                                            \
-    else                                                         \
-    {                                                            \
-        _src = &_entry->fe_u_src.fe_src;                         \
-        if (_src->fes_flags & FIB_ENTRY_SRC_FLAG_ADDED) {        \
-            _source = _src->fes_src;                             \
-            do {                                                 \
-                action;                                          \
-            } while(0);                                          \
-        }                                                        \
-    }                                                            \
-}
-
-#define FOR_EACH_SRC(_entry, _src, _source, action)              \
-{                                                                \
-    if (fib_entry_has_multiple_srcs(_entry))                     \
-    {                                                            \
-        vec_foreach(_src, _entry->fe_u_src.fe_srcs)              \
-        {                                                        \
-            _source = _src->fes_src;                             \
-            do {                                                 \
-                action;                                          \
-            } while(0);                                          \
-        }                                                        \
-    }                                                            \
-    else                                                         \
-    {                                                            \
-        _src = &_entry->fe_u_src.fe_src;                         \
-        _source = _src->fes_src;                                 \
-        do {                                                     \
-            action;                                              \
-        } while(0);                                              \
-    }                                                            \
+#define FOR_EACH_SRC_ADDED(_entry, _src, _source, action)	\
+{								\
+    vec_foreach(_src, _entry->fe_srcs)				\
+    {								\
+	if (_src->fes_flags & FIB_ENTRY_SRC_FLAG_ADDED) {	\
+	    _source = _src->fes_src;				\
+	    do {						\
+		action;						\
+	    } while(0);						\
+	}							\
+    }								\
 }
 
 extern u8* fib_entry_src_format(fib_entry_t *entry,
@@ -285,7 +260,7 @@ extern fib_entry_src_flag_t fib_entry_src_action_path_remove(fib_entry_t *fib_en
 							     fib_source_t source,
 							     const fib_route_path_t *path);
 
-extern void fib_entry_src_action_installed(fib_entry_t *fib_entry,
+extern void fib_entry_src_action_installed(const fib_entry_t *fib_entry,
 					   fib_source_t source);
 
 extern fib_forward_chain_type_t fib_entry_get_default_chain_type(
@@ -304,7 +279,6 @@ extern void fib_entry_src_mk_lb (fib_entry_t *fib_entry,
 
 extern fib_protocol_t fib_entry_get_proto(const fib_entry_t * fib_entry);
 extern dpo_proto_t fib_entry_get_dpo_proto(const fib_entry_t * fib_entry);
-extern u32 fib_entry_has_multiple_srcs(const fib_entry_t * fib_entry);
 
 /*
  * Per-source registration. declared here so we save a separate .h file for each
diff --git a/src/vnet/fib/fib_entry_src_mpls.c b/src/vnet/fib/fib_entry_src_mpls.c
index f95ca657705..f80d42afbb0 100644
--- a/src/vnet/fib/fib_entry_src_mpls.c
+++ b/src/vnet/fib/fib_entry_src_mpls.c
@@ -181,6 +181,13 @@ const static fib_entry_src_vft_t mpls_src_vft = {
     .fesv_format = fib_entry_src_mpls_format,
     .fesv_set_data = fib_entry_src_mpls_set_data,
     .fesv_get_data = fib_entry_src_mpls_get_data,
+    /*
+     * .fesv_fwd_update = fib_entry_src_mpls_fwd_update,
+     *  When the forwarding for the IP entry is updated, any MPLS chains
+     * it has created are also updated. Since the MPLS entry will have already
+     * installed that chain/load-balance there is no need to update the netry
+     * FIXME: later: propagate any walk to the children of the MPLS entry. for SR
+     */
 };
 
 void