Re: [ovs-dev] [PATCH 2/2] netdev-offload-dpdk: Fix vxlan vni cast-align warnings

[Eli Britstein via dev]

On 7/22/2021 4:10 PM, Ilya Maximets wrote:
External email: Use caution opening links or attachments

On 7/22/21 3:00 PM, Eli Britstein wrote:
On 7/22/2021 3:28 PM, Ilya Maximets wrote:
External email: Use caution opening links or attachments


On 7/11/21 7:15 AM, Eli Britstein wrote:
Compiling with -Werror and -Wcast-align has errors like:

lib/netdev-offload-dpdk.c: In function 'dump_flow_pattern':
lib/netdev-offload-dpdk.c:385:38: error: cast increases required alignment
      of target type [-Werror=cast-align]
    385 |    ntohl(*(ovs_be32 *) vxlan_spec->vni) >> 8,
        |           ^

Fix them.

Reported-by: Harry Van Haaren <harry.van.haa...@intel.com>
Fixes: 4e432d6f8128 ("netdev-offload-dpdk: Support tnl/push using vxlan encap 
attribute.")
Fixes: e098c2f966cb ("netdev-dpdk-offload: Add vxlan pattern matching 
function.")
Signed-off-by: Eli Britstein <el...@nvidia.com>
---
   lib/netdev-offload-dpdk.c | 18 +++++++++++++-----
   1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/lib/netdev-offload-dpdk.c b/lib/netdev-offload-dpdk.c
index a24f92782..e4b19ae40 100644
--- a/lib/netdev-offload-dpdk.c
+++ b/lib/netdev-offload-dpdk.c
@@ -375,6 +375,8 @@ dump_flow_pattern(struct ds *s,
       } else if (item->type == RTE_FLOW_ITEM_TYPE_VXLAN) {
           const struct rte_flow_item_vxlan *vxlan_spec = item->spec;
           const struct rte_flow_item_vxlan *vxlan_mask = item->mask;
+        BUILD_ASSERT_DECL(offsetof(struct rte_flow_item_vxlan, vni) %
+                          sizeof(ovs_be32) == 0);

           ds_put_cstr(s, "vxlan ");
           if (vxlan_spec) {
@@ -382,8 +384,10 @@ dump_flow_pattern(struct ds *s,
                   vxlan_mask = &rte_flow_item_vxlan_mask;
               }
               DUMP_PATTERN_ITEM(vxlan_mask->vni, "vni", "%"PRIu32,
-                              ntohl(*(ovs_be32 *) vxlan_spec->vni) >> 8,
-                              ntohl(*(ovs_be32 *) vxlan_mask->vni) >> 8);
+                              ntohl(*ALIGNED_CAST(ovs_be32 *,
+                                                  vxlan_spec->vni)) >> 8,
+                              ntohl(*ALIGNED_CAST(ovs_be32 *,
+                                                  vxlan_mask->vni)) >> 8);
           }
           ds_put_cstr(s, "/ ");
       } else {
@@ -417,8 +421,10 @@ dump_vxlan_encap(struct ds *s, const struct rte_flow_item 
*items)
       ds_put_format(s, "set vxlan ip-version %s ",
                     ipv4 ? "ipv4" : ipv6 ? "ipv6" : "ERR");
       if (vxlan) {
+        BUILD_ASSERT_DECL(offsetof(struct rte_flow_item_vxlan, vni) %
+                          sizeof(ovs_be32) == 0);
           ds_put_format(s, "vni %"PRIu32" ",
-                      ntohl(*(ovs_be32 *) vxlan->vni) >> 8);
+                      ntohl(*ALIGNED_CAST(ovs_be32 *, vxlan->vni)) >> 8);
       }
       if (udp) {
           ds_put_format(s, "udp-src %"PRIu16" udp-dst %"PRIu16" ",
@@ -1003,9 +1009,11 @@ parse_vxlan_match(struct flow_patterns *patterns,
       vx_spec = xzalloc(sizeof *vx_spec);
       vx_mask = xzalloc(sizeof *vx_mask);

-    put_unaligned_be32((ovs_be32 *) vx_spec->vni,
+    BUILD_ASSERT_DECL(offsetof(struct rte_flow_item_vxlan, vni) %
+                      sizeof(ovs_be32) == 0);
+    put_unaligned_be32(ALIGNED_CAST(ovs_be32 *, vx_spec->vni),
                          htonl(ntohll(match->flow.tunnel.tun_id) << 8));
-    put_unaligned_be32((ovs_be32 *) vx_mask->vni,
+    put_unaligned_be32(ALIGNED_CAST(ovs_be32 *, vx_mask->vni),
                          htonl(ntohll(match->wc.masks.tunnel.tun_id) << 8));

       consumed_masks->tunnel.tun_id = 0;

Same concerns here about the build time assertion as in the patch #1.
It also seems redundant to use put_unaligned_* functions and have a
build assertion at the same time.

Suggesting to just use put/get_unaligned_* in all cases and remove
build time assertions.
The code before this patch just uses (for example) put_unaligned_be32, which 
its 1st argument is (ovs_be32 *).

vni in struct rte_flow_item_vxlan  in dpdk is uint8_t vni[3]; /**< VXLAN 
identifier. */

I use ALIGNED_CAST to mute the warning, and the assert to make sure the 
alignment is correct.

I don't understand your suggestion here, unless you suggest to use memcpy as 
suggested in patch#1.
put_unaligned_be32 is an alias for put_unaligned_u32 that
is implemented like this:

116 static inline void put_unaligned_u32(uint32_t *p_, uint32_t x_)
117 {
118     uint8_t *p = (uint8_t *) p_;
119     uint32_t x = ntohl(x_);
120
121     p[0] = x >> 24;
122     p[1] = x >> 16;
123     p[2] = x >> 8;
124     p[3] = x;
125 }

or by the equivalent function provided by compiler.

The memory copy performed byte-by-byte, hence independent form the
original alignment of the memory.  So, you may add ALIGNED_CAST to
silence the warning, but we don't need the build assertion, because
put_unaligned_* functions requires 1 byte alignment which is always
there.
Removing the asserts implies knowledge of the internal implementation of 
put_unaligned_u32 by another file.

Best regards, Ilya Maximets.
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