Re: Possible bug? DOM-U network stopped working after fatal error reported in DOM0

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Dec 29, 2021 at 11:27:50AM +0100, Roger Pau Monné wrote:
> On Wed, Dec 29, 2021 at 05:13:00PM +0800, G.R. wrote:
> > >
> > > I think this is hitting a KASSERT, could you paste the text printed as
> > > part of the panic (not just he backtrace)?
> > >
> > > Sorry this is taking a bit of time to solve.
> > >
> > > Thanks!
> > >
> > Sorry that I didn't make it clear in the first place.
> > It is the same cross boundary assertion.
> 
> I see. After looking at the code it seems like sglist will coalesce
> contiguous physical ranges without taking page boundaries into
> account, which is not suitable for our purpose here. I guess I will
> either have to modify sglist, or switch to using bus_dma. The main
> problem with using bus_dma is that it will require bigger changes to
> netfront I think.

I have a crappy patch to use bus_dma. It's not yet ready for upstream
but you might want to give it a try to see if it solves the cross page
boundary issues.

Thanks, Roger.
---
diff --git a/sys/dev/xen/netfront/netfront.c b/sys/dev/xen/netfront/netfront.c
index 8dba5a8dc6d5..693ef25c8783 100644
--- a/sys/dev/xen/netfront/netfront.c
+++ b/sys/dev/xen/netfront/netfront.c
@@ -71,6 +71,8 @@ __FBSDID("$FreeBSD$");
 #include <xen/interface/io/netif.h>
 #include <xen/xenbus/xenbusvar.h>
 
+#include <machine/bus.h>
+
 #include "xenbus_if.h"
 
 /* Features supported by all backends.  TSO and LRO can be negotiated */
@@ -199,6 +201,12 @@ struct netfront_txq {
        struct taskqueue        *tq;
        struct task             defrtask;
 
+       bus_dmamap_t            dma_map;
+       struct mbuf_refcount {
+               struct m_tag    tag;
+               u_int           count;
+       }                       refcount_tag[NET_TX_RING_SIZE + 1];
+
        bool                    full;
 };
 
@@ -221,6 +229,8 @@ struct netfront_info {
 
        struct ifmedia          sc_media;
 
+       bus_dma_tag_t           dma_tag;
+
        bool                    xn_reset;
 };
 
@@ -301,6 +311,39 @@ xn_get_rx_ref(struct netfront_rxq *rxq, RING_IDX ri)
        return (ref);
 }
 
+#define MTAG_COOKIE 1218492000
+#define MTAG_REFCOUNT 0
+
+static void mbuf_grab(struct mbuf *m)
+{
+       struct mbuf_refcount *ref;
+
+       ref = (struct mbuf_refcount *)m_tag_locate(m, MTAG_COOKIE,
+           MTAG_REFCOUNT, NULL);
+       KASSERT(ref != NULL, ("Cannot find refcount"));
+       ref->count++;
+}
+
+static void mbuf_release(struct mbuf *m)
+{
+       struct mbuf_refcount *ref;
+
+       ref = (struct mbuf_refcount *)m_tag_locate(m, MTAG_COOKIE,
+           MTAG_REFCOUNT, NULL);
+       KASSERT(ref != NULL, ("Cannot find refcount"));
+       KASSERT(ref->count > 0, ("Invalid reference count"));
+
+       if (--ref->count == 0)
+               m_freem(m);
+}
+
+static void tag_free(struct m_tag *t)
+{
+       struct mbuf_refcount *ref = (struct mbuf_refcount *)t;
+
+       KASSERT(ref->count == 0, ("Free mbuf tag with pending refcnt"));
+}
+
 #define IPRINTK(fmt, args...) \
     printf("[XEN] " fmt, ##args)
 #ifdef INVARIANTS
@@ -783,6 +826,7 @@ destroy_txq(struct netfront_txq *txq)
        buf_ring_free(txq->br, M_DEVBUF);
        taskqueue_drain_all(txq->tq);
        taskqueue_free(txq->tq);
+       bus_dmamap_destroy(txq->info->dma_tag, txq->dma_map);
 }
 
 static void
@@ -826,6 +870,11 @@ setup_txqs(device_t dev, struct netfront_info *info,
                for (i = 0; i <= NET_TX_RING_SIZE; i++) {
                        txq->mbufs[i] = (void *) ((u_long) i+1);
                        txq->grant_ref[i] = GRANT_REF_INVALID;
+                       m_tag_setup(&txq->refcount_tag[i].tag,
+                           MTAG_COOKIE, MTAG_REFCOUNT,
+                           sizeof(txq->refcount_tag[i]) -
+                           sizeof(txq->refcount_tag[i].tag));
+                       txq->refcount_tag[i].tag.m_tag_free = &tag_free;
                }
                txq->mbufs[NET_TX_RING_SIZE] = (void *)0;
 
@@ -874,10 +923,18 @@ setup_txqs(device_t dev, struct netfront_info *info,
                        device_printf(dev, "xen_intr_alloc_and_bind_local_port 
failed\n");
                        goto fail_bind_port;
                }
+
+               error = bus_dmamap_create(info->dma_tag, 0, &txq->dma_map);
+               if (error != 0) {
+                       device_printf(dev, "failed to create dma map\n");
+                       goto fail_dma_map;
+               }
        }
 
        return (0);
 
+fail_dma_map:
+       xen_intr_unbind(&txq->xen_intr_handle);
 fail_bind_port:
        taskqueue_drain_all(txq->tq);
 fail_start_thread:
@@ -1041,7 +1098,7 @@ xn_release_tx_bufs(struct netfront_txq *txq)
                if (txq->mbufs_cnt < 0) {
                        panic("%s: tx_chain_cnt must be >= 0", __func__);
                }
-               m_free(m);
+               mbuf_release(m);
        }
 }
 
@@ -1311,7 +1368,7 @@ xn_txeof(struct netfront_txq *txq)
                        txq->mbufs[id] = NULL;
                        add_id_to_freelist(txq->mbufs, id);
                        txq->mbufs_cnt--;
-                       m_free(m);
+                       mbuf_release(m);
                        /* Only mark the txq active if we've freed up at least 
one slot to try */
                        ifp->if_drv_flags &= ~IFF_DRV_OACTIVE;
                }
@@ -1530,27 +1587,18 @@ xn_count_frags(struct mbuf *m)
 static int
 xn_assemble_tx_request(struct netfront_txq *txq, struct mbuf *m_head)
 {
-       struct mbuf *m;
        struct netfront_info *np = txq->info;
        struct ifnet *ifp = np->xn_ifp;
-       u_int nfrags;
-       int otherend_id;
+       int otherend_id, error, nfrags;
+       unsigned int i;
+       bus_dma_segment_t segs[MAX_TX_REQ_FRAGS];
 
-       /**
-        * Defragment the mbuf if necessary.
-        */
-       nfrags = xn_count_frags(m_head);
+       error = bus_dmamap_load_mbuf_sg(np->dma_tag, txq->dma_map, m_head,
+           segs, &nfrags, 0);
+       if (error == EFBIG || nfrags > np->maxfrags) {
+               struct mbuf *m;
 
-       /*
-        * Check to see whether this request is longer than netback
-        * can handle, and try to defrag it.
-        */
-       /**
-        * It is a bit lame, but the netback driver in Linux can't
-        * deal with nfrags > MAX_TX_REQ_FRAGS, which is a quirk of
-        * the Linux network stack.
-        */
-       if (nfrags > np->maxfrags) {
+               bus_dmamap_unload(np->dma_tag, txq->dma_map);
                m = m_defrag(m_head, M_NOWAIT);
                if (!m) {
                        /*
@@ -1561,15 +1609,18 @@ xn_assemble_tx_request(struct netfront_txq *txq, struct 
mbuf *m_head)
                        return (EMSGSIZE);
                }
                m_head = m;
+               error = bus_dmamap_load_mbuf_sg(np->dma_tag, txq->dma_map,
+                   m_head, segs, &nfrags, 0);
+               if (error != 0 || nfrags > np->maxfrags) {
+                       bus_dmamap_unload(np->dma_tag, txq->dma_map);
+                       m_freem(m_head);
+                       return (error ?: EFBIG);
+               }
+       } else if (error != 0) {
+               m_freem(m_head);
+               return (error);
        }
 
-       /* Determine how many fragments now exist */
-       nfrags = xn_count_frags(m_head);
-
-       /*
-        * Check to see whether the defragmented packet has too many
-        * segments for the Linux netback driver.
-        */
        /**
         * The FreeBSD TCP stack, with TSO enabled, can produce a chain
         * of mbufs longer than Linux can handle.  Make sure we don't
@@ -1604,9 +1655,8 @@ xn_assemble_tx_request(struct netfront_txq *txq, struct 
mbuf *m_head)
         * the fragment pointers. Stop when we run out
         * of fragments or hit the end of the mbuf chain.
         */
-       m = m_head;
        otherend_id = xenbus_get_otherend_id(np->xbdev);
-       for (m = m_head; m; m = m->m_next) {
+       for (i = 0; i < nfrags; i++) {
                netif_tx_request_t *tx;
                uintptr_t id;
                grant_ref_t ref;
@@ -1621,17 +1671,22 @@ xn_assemble_tx_request(struct netfront_txq *txq, struct 
mbuf *m_head)
                if (txq->mbufs_cnt > NET_TX_RING_SIZE)
                        panic("%s: tx_chain_cnt must be <= NET_TX_RING_SIZE\n",
                            __func__);
-               txq->mbufs[id] = m;
+               if (i == 0)
+                       m_tag_prepend(m_head, &txq->refcount_tag[id].tag);
+               mbuf_grab(m_head);
+               txq->mbufs[id] = m_head;
                tx->id = id;
                ref = gnttab_claim_grant_reference(&txq->gref_head);
                KASSERT((short)ref >= 0, ("Negative ref"));
-               mfn = virt_to_mfn(mtod(m, vm_offset_t));
+               mfn = atop(segs[i].ds_addr);
                gnttab_grant_foreign_access_ref(ref, otherend_id,
                    mfn, GNTMAP_readonly);
                tx->gref = txq->grant_ref[id] = ref;
-               tx->offset = mtod(m, vm_offset_t) & (PAGE_SIZE - 1);
+               tx->offset = segs[i].ds_addr & PAGE_MASK;
+               KASSERT(tx->offset + segs[i].ds_len <= PAGE_SIZE,
+                   ("mbuf segment crosses a page boundary"));
                tx->flags = 0;
-               if (m == m_head) {
+               if (i == 0) {
                        /*
                         * The first fragment has the entire packet
                         * size, subsequent fragments have just the
@@ -1640,7 +1695,7 @@ xn_assemble_tx_request(struct netfront_txq *txq, struct 
mbuf *m_head)
                         * subtracting the sizes of the other
                         * fragments.
                         */
-                       tx->size = m->m_pkthdr.len;
+                       tx->size = m_head->m_pkthdr.len;
 
                        /*
                         * The first fragment contains the checksum flags
@@ -1654,12 +1709,12 @@ xn_assemble_tx_request(struct netfront_txq *txq, struct 
mbuf *m_head)
                         * so we have to test for CSUM_TSO
                         * explicitly.
                         */
-                       if (m->m_pkthdr.csum_flags
+                       if (m_head->m_pkthdr.csum_flags
                            & (CSUM_DELAY_DATA | CSUM_TSO)) {
                                tx->flags |= (NETTXF_csum_blank
                                    | NETTXF_data_validated);
                        }
-                       if (m->m_pkthdr.csum_flags & CSUM_TSO) {
+                       if (m_head->m_pkthdr.csum_flags & CSUM_TSO) {
                                struct netif_extra_info *gso =
                                        (struct netif_extra_info *)
                                        RING_GET_REQUEST(&txq->ring,
@@ -1667,7 +1722,7 @@ xn_assemble_tx_request(struct netfront_txq *txq, struct 
mbuf *m_head)
 
                                tx->flags |= NETTXF_extra_info;
 
-                               gso->u.gso.size = m->m_pkthdr.tso_segsz;
+                               gso->u.gso.size = m_head->m_pkthdr.tso_segsz;
                                gso->u.gso.type =
                                        XEN_NETIF_GSO_TYPE_TCPV4;
                                gso->u.gso.pad = 0;
@@ -1677,13 +1732,14 @@ xn_assemble_tx_request(struct netfront_txq *txq, struct 
mbuf *m_head)
                                gso->flags = 0;
                        }
                } else {
-                       tx->size = m->m_len;
+                       tx->size = segs[i].ds_len;
                }
-               if (m->m_next)
+               if (i != nfrags - 1)
                        tx->flags |= NETTXF_more_data;
 
                txq->ring.req_prod_pvt++;
        }
+       bus_dmamap_unload(np->dma_tag, txq->dma_map);
        BPF_MTAP(ifp, m_head);
 
        if_inc_counter(ifp, IFCOUNTER_OPACKETS, 1);
@@ -2244,7 +2300,20 @@ create_netdev(device_t dev)
        ether_ifattach(ifp, np->mac);
        netfront_carrier_off(np);
 
-       return (0);
+       err = bus_dma_tag_create(
+           bus_get_dma_tag(dev),               /* parent */
+           1, PAGE_SIZE,                       /* algnmnt, boundary */
+           BUS_SPACE_MAXADDR,                  /* lowaddr */
+           BUS_SPACE_MAXADDR,                  /* highaddr */
+           NULL, NULL,                         /* filter, filterarg */
+           PAGE_SIZE * MAX_TX_REQ_FRAGS,       /* max request size */
+           MAX_TX_REQ_FRAGS,                   /* max segments */
+           PAGE_SIZE,                          /* maxsegsize */
+           BUS_DMA_ALLOCNOW,                   /* flags */
+           NULL, NULL,                         /* lockfunc, lockarg */
+           &np->dma_tag);
+
+       return (err);
 
 error:
        KASSERT(err != 0, ("Error path with no error code specified"));
@@ -2277,6 +2346,7 @@ netif_free(struct netfront_info *np)
        if_free(np->xn_ifp);
        np->xn_ifp = NULL;
        ifmedia_removeall(&np->sc_media);
+       bus_dma_tag_destroy(np->dma_tag);
 }
 
 static void