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Hi,
can you try the attached patch ? obviously m_copyback() doesn't work
the way I though it was, and as it's not even capable of allocating
a cluster when needeed it's probably less efficient than plain MCLGET/memcpy.

-- 
Manuel Bouyer <bouyer@antioche.eu.org>
     NetBSD: 26 ans d'experience feront toujours la difference
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Index: if_xennet.c
===================================================================
RCS file: /cvsroot/src/sys/arch/xen/xen/if_xennet.c,v
retrieving revision 1.43
diff -u -r1.43 if_xennet.c
--- if_xennet.c	6 Mar 2006 22:04:18 -0000	1.43
+++ if_xennet.c	16 Mar 2006 20:18:49 -0000
@@ -787,16 +787,21 @@
 			 * memory, copy data and push the receive
 			 * buffer back to the hypervisor.
 			 */
-			m->m_len = MHLEN;
-			m->m_pkthdr.len = 0;
-			m_copyback(m, 0, rx->status, pktp);
-			xennet_rx_push_buffer(sc, rx->id);
-			if (m->m_pkthdr.len < rx->status) {
-				/* out of memory, just drop packets */
-				ifp->if_ierrors++;
-				m_freem(m);
-				continue;
+			if (rx->status > MHLEN) {
+				MCLGET(m, M_DONTWAIT);
+				if (__predict_false(
+				    (m->m_flags & M_EXT) == 0)) {
+					/* out of memory, just drop packets */
+					ifp->if_ierrors++;
+					m_freem(m);
+					xennet_rx_push_buffer(sc, rx->id);
+					continue;
+				}
 			}
+					
+			m->m_len = m->m_pkthdr.len = rx->status;
+			memcpy(mtod(m, void *), pktp, rx->status);
+			xennet_rx_push_buffer(sc, rx->id);
 		}
 
 #ifdef XENNET_DEBUG_DUMP

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