flow: add miniflow_pad_from_64

Provide leading padding to allow pushing a value to a miniflow where
the value is not aligned to 64 bytes and no value has already been
pushed to the same word.

This will be used by a follow-up patch to allow layer 3 packet - that is
packets without an ethernet header - to be represented in flows.

Signed-off-by: Simon Horman <simon.horman@netronome.com>
Acked-by: Jarno Rajahalme <jarno@ovn.org>

lib/flow.c

@@ -225,6 +225,16 @@ BUILD_MESSAGE("FLOW_WC_SEQ changed: miniflow_extract() will have runtime "
     MF.data++;                                                  \
 }
 
+#define miniflow_pad_from_64_(MF, OFS)                          \
+{                                                               \
+    MINIFLOW_ASSERT(MF.data < MF.end);                          \
+                                                                \
+    MINIFLOW_ASSERT((OFS) % 8 != 0);                            \
+    miniflow_set_map(MF, OFS / 8);                              \
+                                                                \
+    memset((uint8_t *)MF.data, 0, (OFS) % 8);                   \
+}
+
 #define miniflow_push_be16_(MF, OFS, VALUE)                     \
     miniflow_push_uint16_(MF, OFS, (OVS_FORCE uint16_t)VALUE);
 
@@ -288,6 +298,9 @@ BUILD_MESSAGE("FLOW_WC_SEQ changed: miniflow_extract() will have runtime "
 #define miniflow_pad_to_64(MF, FIELD)                       \
     miniflow_pad_to_64_(MF, OFFSETOFEND(struct flow, FIELD))
 
+#define miniflow_pad_from_64(MF, FIELD)                       \
+    miniflow_pad_from_64_(MF, offsetof(struct flow, FIELD))
+
 #define miniflow_push_words(MF, FIELD, VALUEP, N_WORDS)                 \
     miniflow_push_words_(MF, offsetof(struct flow, FIELD), VALUEP, N_WORDS)