Revert m_defrag() mbuf alignment to IP header.

m_defrag() is intended as last resort to make DMA transfers to the
hardware.  Therefore page alingment is more important than IP header
alignment.  The reason, why the mbuf returned by m_defrag() was
switched to IP header alingment, was that ether_extract_headers()
failed in em(4) driver with TSO on sparc64.  This has been fixed
by using memcpy().

The alignment change in m_defrag() is too late in the 7.5 relaese
process.  It may affect several drivers on different architectures.
Bus dmamap for ixl(4) on sun4v expects page alignment.  Such alignment
issues and TSO mbuf mapping for IOMMU need more thought.

OK deraadt@

diff --git a/sys/kern/uipc_mbuf.c b/sys/kern/uipc_mbuf.c
index ec675a2..fc7b515 100644 (file)
--- a/sys/kern/uipc_mbuf.c
+++ b/sys/kern/uipc_mbuf.c
@@ -1,4 +1,4 @@
-/*     $OpenBSD: uipc_mbuf.c,v 1.289 2024/02/21 09:28:29 claudio Exp $ */
+/*     $OpenBSD: uipc_mbuf.c,v 1.290 2024/03/05 18:52:41 bluhm Exp $   */
 /*     $NetBSD: uipc_mbuf.c,v 1.15.4.1 1996/06/13 17:11:44 cgd Exp $   */
 
 /*
@@ -545,29 +545,27 @@ m_purge(struct mbuf *m)
  * mbuf chain defragmenter. This function uses some evil tricks to defragment
  * an mbuf chain into a single buffer without changing the mbuf pointer.
  * This needs to know a lot of the mbuf internals to make this work.
+ * The resulting mbuf is not aligned to IP header to assist DMA transfers.
  */
 int
 m_defrag(struct mbuf *m, int how)
 {
        struct mbuf *m0;
-       unsigned int adj;
 
        if (m->m_next == NULL)
                return (0);
 
        KASSERT(m->m_flags & M_PKTHDR);
 
-       adj = mtod(m, unsigned long) & (sizeof(long) - 1);
        if ((m0 = m_gethdr(how, m->m_type)) == NULL)
                return (ENOBUFS);
-       if (m->m_pkthdr.len + adj > MHLEN) {
-               MCLGETL(m0, how, m->m_pkthdr.len + adj);
+       if (m->m_pkthdr.len > MHLEN) {
+               MCLGETL(m0, how, m->m_pkthdr.len);
                if (!(m0->m_flags & M_EXT)) {
                        m_free(m0);
                        return (ENOBUFS);
                }
        }
-       m0->m_data += adj;
        m_copydata(m, 0, m->m_pkthdr.len, mtod(m0, caddr_t));
        m0->m_pkthdr.len = m0->m_len = m->m_pkthdr.len;
 
@@ -586,9 +584,9 @@ m_defrag(struct mbuf *m, int how)
                memcpy(&m->m_ext, &m0->m_ext, sizeof(struct mbuf_ext));
                MCLINITREFERENCE(m);
                m->m_flags |= m0->m_flags & (M_EXT|M_EXTWR);
-               m->m_data = m->m_ext.ext_buf + adj;
+               m->m_data = m->m_ext.ext_buf;
        } else {
-               m->m_data = m->m_pktdat + adj;
+               m->m_data = m->m_pktdat;
                memcpy(m->m_data, m0->m_data, m0->m_len);
        }
        m->m_pkthdr.len = m->m_len = m0->m_len;