xen/netfront: Add 2 bytes padding in the rx mbuf

The ethernet header size is not word aligned. Therefore the IP packet and so
on won't be align. On some architecture (such as ARM) unaligned access may
be slower and/or not supported. Therefore we might reveice an alignement
fault. To avoid this case, we need to pull-up the data of ETHER_ALIGN bytes.

I'm not sure how this patch will impact x86, we need to do some benchmarking
without and with it.

Furthermore, I don't know if m_copyup is the rigth function to use here. Can
any expert to the network stack can tell me if there is a better solution?

diff --git a/sys/dev/xen/netfront/netfront.c b/sys/dev/xen/netfront/netfront.c
index 7588956210e6e4ba8285687c15560926101af518..57f826f1d0cce0cb8803f741b1fc3bc46d03d6da 100644 (file)
--- a/sys/dev/xen/netfront/netfront.c
+++ b/sys/dev/xen/netfront/netfront.c
@@ -86,6 +86,8 @@ __FBSDID("$FreeBSD$");
 
 #include "xenbus_if.h"
 
+#define ETHER_ALIGN    2
+
 /* Features supported by all backends.  TSO and LRO can be negotiated */
 #define XN_CSUM_FEATURES       (CSUM_TCP | CSUM_UDP)
 
@@ -1247,6 +1249,22 @@ next_skip_queue:
                ref_cons = *cons + frags;
                frags++;
        }
+
+       if (err == 0) {
+               /*
+                * The ethernet header size is not word aligned. Therefore,
+                * the IP packet and so on won't be align. On some architecture
+                * (such as ARM) unaligned access may be slower and/or not
+                * supported. Therefore we might receive an alignment fault.
+                * To avoid this case, we need to pull-up the data of
+                * ETHER_ALIGN bytes.
+                */
+               m0 = m_copyup(m0, min(m0->m_pkthdr.len, max_protohdr),
+                             ETHER_ALIGN);
+               if (m0 == 0)
+                       err = ENOMEM;
+       }
+
        *list = m0;
        *cons += frags;