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[Xen-devel] [9/11] [NET] back: Transmit SG packets if supported

To: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>, Xen Development Mailing List <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: [Xen-devel] [9/11] [NET] back: Transmit SG packets if supported
From: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Date: Sat, 8 Jul 2006 00:20:54 +1000
Delivery-date: Fri, 07 Jul 2006 07:23:53 -0700
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Hi:

[NET] back: Transmit SG packets if supported

This patch adds scatter-and-gather transmission support to the backend.
This allows the MTU to be raised right now and the potential for TSO in
future.

Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@xxxxxxxxxxxxxxxxxxx>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
--
diff -r c82398c08669 -r 54f84d198a1d 
linux-2.6-xen-sparse/drivers/xen/netback/common.h
--- a/linux-2.6-xen-sparse/drivers/xen/netback/common.h Fri Jul 07 23:37:45 
2006 +1000
+++ b/linux-2.6-xen-sparse/drivers/xen/netback/common.h Fri Jul 07 23:38:49 
2006 +1000
@@ -130,4 +130,10 @@ static inline int netbk_can_queue(struct
        return netif->features & NETIF_F_LLTX;
 }
 
+static inline int netbk_can_sg(struct net_device *dev)
+{
+       netif_t *netif = netdev_priv(dev);
+       return netif->features & NETIF_F_SG;
+}
+
 #endif /* __NETIF__BACKEND__COMMON_H__ */
diff -r c82398c08669 -r 54f84d198a1d 
linux-2.6-xen-sparse/drivers/xen/netback/interface.c
--- a/linux-2.6-xen-sparse/drivers/xen/netback/interface.c      Fri Jul 07 
23:37:45 2006 +1000
+++ b/linux-2.6-xen-sparse/drivers/xen/netback/interface.c      Fri Jul 07 
23:38:49 2006 +1000
@@ -68,10 +68,34 @@ static int net_close(struct net_device *
        return 0;
 }
 
+static int netbk_change_mtu(struct net_device *dev, int mtu)
+{
+       int max = netbk_can_sg(dev) ? 65535 - ETH_HLEN : ETH_DATA_LEN;
+
+       if (mtu > max)
+               return -EINVAL;
+       dev->mtu = mtu;
+       return 0;
+}
+
+static int netbk_set_sg(struct net_device *dev, u32 data)
+{
+       if (data) {
+               netif_t *netif = netdev_priv(dev);
+
+               if (!(netif->features & NETIF_F_SG))
+                       return -ENOSYS;
+       }
+
+       return ethtool_op_set_sg(dev, data);
+}
+
 static struct ethtool_ops network_ethtool_ops =
 {
        .get_tx_csum = ethtool_op_get_tx_csum,
        .set_tx_csum = ethtool_op_set_tx_csum,
+       .get_sg = ethtool_op_get_sg,
+       .set_sg = netbk_set_sg,
 };
 
 netif_t *netif_alloc(domid_t domid, unsigned int handle, u8 be_mac[ETH_ALEN])
@@ -107,6 +131,7 @@ netif_t *netif_alloc(domid_t domid, unsi
        dev->get_stats       = netif_be_get_stats;
        dev->open            = net_open;
        dev->stop            = net_close;
+       dev->change_mtu      = netbk_change_mtu;
        dev->features        = NETIF_F_IP_CSUM;
 
        SET_ETHTOOL_OPS(dev, &network_ethtool_ops);
diff -r c82398c08669 -r 54f84d198a1d 
linux-2.6-xen-sparse/drivers/xen/netback/netback.c
--- a/linux-2.6-xen-sparse/drivers/xen/netback/netback.c        Fri Jul 07 
23:37:45 2006 +1000
+++ b/linux-2.6-xen-sparse/drivers/xen/netback/netback.c        Fri Jul 07 
23:38:49 2006 +1000
@@ -40,6 +40,11 @@
 
 /*#define NETBE_DEBUG_INTERRUPT*/
 
+struct netbk_rx_meta {
+       skb_frag_t frag;
+       int id;
+};
+
 static void netif_idx_release(u16 pending_idx);
 static void netif_page_release(struct page *page);
 static void make_tx_response(netif_t *netif, 
@@ -100,21 +105,27 @@ static unsigned long mfn_list[MAX_MFN_AL
 static unsigned long mfn_list[MAX_MFN_ALLOC];
 static unsigned int alloc_index = 0;
 
-static unsigned long alloc_mfn(void)
-{
-       unsigned long mfn = 0;
+static inline unsigned long alloc_mfn(void)
+{
+       return mfn_list[--alloc_index];
+}
+
+static int check_mfn(int nr)
+{
        struct xen_memory_reservation reservation = {
-               .nr_extents   = MAX_MFN_ALLOC,
                .extent_order = 0,
                .domid        = DOMID_SELF
        };
-       set_xen_guest_handle(reservation.extent_start, mfn_list);
-       if ( unlikely(alloc_index == 0) )
-               alloc_index = HYPERVISOR_memory_op(
-                       XENMEM_increase_reservation, &reservation);
-       if ( alloc_index != 0 )
-               mfn = mfn_list[--alloc_index];
-       return mfn;
+
+       if (likely(alloc_index >= nr))
+               return 0;
+
+       set_xen_guest_handle(reservation.extent_start, mfn_list + alloc_index);
+       reservation.nr_extents = MAX_MFN_ALLOC - alloc_index;
+       alloc_index += HYPERVISOR_memory_op(XENMEM_increase_reservation,
+                                           &reservation);
+
+       return alloc_index >= nr ? 0 : -ENOMEM;
 }
 
 static inline void maybe_schedule_tx_action(void)
@@ -136,12 +147,87 @@ static inline int is_xen_skb(struct sk_b
        return (cp == skbuff_cachep);
 }
 
+static struct sk_buff *netbk_copy_skb(struct sk_buff *skb)
+{
+       struct skb_shared_info *ninfo;
+       struct sk_buff *nskb;
+       unsigned long offset;
+       int ret;
+       int len;
+       int headlen;
+
+       nskb = alloc_skb(SKB_MAX_HEAD(0), GFP_ATOMIC);
+       if (unlikely(!nskb))
+               goto err;
+
+       skb_reserve(nskb, 16);
+       headlen = nskb->end - nskb->data;
+       if (headlen > skb_headlen(skb))
+               headlen = skb_headlen(skb);
+       ret = skb_copy_bits(skb, 0, __skb_put(nskb, headlen), headlen);
+       BUG_ON(ret);
+
+       ninfo = skb_shinfo(nskb);
+       ninfo->gso_size = skb_shinfo(skb)->gso_size;
+       ninfo->gso_type = skb_shinfo(skb)->gso_type;
+
+       offset = headlen;
+       len = skb->len - headlen;
+
+       nskb->len = skb->len;
+       nskb->data_len = len;
+       nskb->truesize += len;
+
+       while (len) {
+               struct page *page;
+               int copy;
+               int zero;
+
+               if (unlikely(ninfo->nr_frags >= MAX_SKB_FRAGS)) {
+                       dump_stack();
+                       goto err_free;
+               }
+
+               copy = len >= PAGE_SIZE ? PAGE_SIZE : len;
+               zero = len >= PAGE_SIZE ? 0 : __GFP_ZERO;
+
+               page = alloc_page(GFP_ATOMIC | zero);
+               if (unlikely(!page))
+                       goto err_free;
+
+               ret = skb_copy_bits(skb, offset, page_address(page), copy);
+               BUG_ON(ret);
+
+               ninfo->frags[ninfo->nr_frags].page = page;
+               ninfo->frags[ninfo->nr_frags].page_offset = 0;
+               ninfo->frags[ninfo->nr_frags].size = copy;
+               ninfo->nr_frags++;
+
+               offset += copy;
+               len -= copy;
+       }
+
+       offset = nskb->data - skb->data;
+
+       nskb->h.raw = skb->h.raw + offset;
+       nskb->nh.raw = skb->nh.raw + offset;
+       nskb->mac.raw = skb->mac.raw + offset;
+
+       return nskb;
+
+err_free:
+       kfree_skb(nskb);
+err:
+       return NULL;
+}
+
 static inline int netbk_queue_full(netif_t *netif)
 {
        RING_IDX peek = netif->rx_req_cons_peek;
 
-       return netif->rx.sring->req_prod - peek <= 0 ||
-              netif->rx.rsp_prod_pvt + NET_RX_RING_SIZE - peek <= 0;
+       return netif->rx.sring->req_prod - peek <= MAX_SKB_FRAGS ||
+              netif->rx.rsp_prod_pvt + NET_RX_RING_SIZE - peek <=
+              MAX_SKB_FRAGS;
 }
 
 int netif_be_start_xmit(struct sk_buff *skb, struct net_device *dev)
@@ -163,20 +249,12 @@ int netif_be_start_xmit(struct sk_buff *
         * We do not copy the packet unless:
         *  1. The data is shared; or
         *  2. The data is not allocated from our special cache.
-        * NB. We also couldn't cope with fragmented packets, but we won't get
-        *     any because we not advertise the NETIF_F_SG feature.
+        *  3. The data is fragmented.
         */
-       if (skb_shared(skb) || skb_cloned(skb) || !is_xen_skb(skb)) {
-               int hlen = skb->data - skb->head;
-               int ret;
-               struct sk_buff *nskb = dev_alloc_skb(hlen + skb->len);
+       if (skb_cloned(skb) || skb_is_nonlinear(skb) || !is_xen_skb(skb)) {
+               struct sk_buff *nskb = netbk_copy_skb(skb);
                if ( unlikely(nskb == NULL) )
                        goto drop;
-               skb_reserve(nskb, hlen);
-               __skb_put(nskb, skb->len);
-               ret = skb_copy_bits(skb, -hlen, nskb->data - hlen,
-                                    skb->len + hlen);
-               BUG_ON(ret);
                /* Copy only the header fields we use in this driver. */
                nskb->dev = skb->dev;
                nskb->ip_summed = skb->ip_summed;
@@ -185,7 +263,7 @@ int netif_be_start_xmit(struct sk_buff *
                skb = nskb;
        }
 
-       netif->rx_req_cons_peek++;
+       netif->rx_req_cons_peek += skb_shinfo(skb)->nr_frags + 1;
        netif_get(netif);
 
        if (netbk_can_queue(dev) && netbk_queue_full(netif))
@@ -221,116 +299,80 @@ int xen_network_done(void)
 }
 #endif
 
-static void net_rx_action(unsigned long unused)
-{
-       netif_t *netif = NULL; 
-       s8 status;
-       u16 size, id, irq, flags;
-       multicall_entry_t *mcl;
-       mmu_update_t *mmu;
-       gnttab_transfer_t *gop;
-       unsigned long vdata, old_mfn, new_mfn;
-       struct sk_buff_head rxq;
-       struct sk_buff *skb;
-       int notify_nr = 0;
-       int ret;
+static u16 netbk_gop_frag(netif_t *netif, struct page *page, int count, int i)
+{
+       multicall_entry_t *mcl = rx_mcl + count;
+       mmu_update_t *mmu = rx_mmu + count;
+       gnttab_transfer_t *gop = grant_rx_op + count;
+       netif_rx_request_t *req;
+       unsigned long old_mfn, new_mfn;
+
+       old_mfn = virt_to_mfn(page_address(page));
+
+       if (!xen_feature(XENFEAT_auto_translated_physmap)) {
+               new_mfn = alloc_mfn();
+
+               /*
+                * Set the new P2M table entry before reassigning
+                * the old data page. Heed the comment in
+                * pgtable-2level.h:pte_page(). :-)
+                */
+               set_phys_to_machine(page_to_pfn(page), new_mfn);
+
+               MULTI_update_va_mapping(mcl, (unsigned long)page_address(page),
+                                       pfn_pte_ma(new_mfn, PAGE_KERNEL), 0);
+
+               mmu->ptr = ((maddr_t)new_mfn << PAGE_SHIFT) |
+                       MMU_MACHPHYS_UPDATE;
+               mmu->val = page_to_pfn(page);
+       }
+
+       req = RING_GET_REQUEST(&netif->rx, netif->rx.req_cons + i);
+       gop->mfn = old_mfn;
+       gop->domid = netif->domid;
+       gop->ref = req->gref;
+       return req->id;
+}
+
+static void netbk_gop_skb(struct sk_buff *skb, struct netbk_rx_meta *meta,
+                         int count)
+{
+       netif_t *netif = netdev_priv(skb->dev);
+       int nr_frags = skb_shinfo(skb)->nr_frags;
+       int i;
+
+       for (i = 0; i < nr_frags; i++) {
+               meta[++count].frag = skb_shinfo(skb)->frags[i];
+               meta[count].id = netbk_gop_frag(netif, meta[count].frag.page,
+                                               count, i + 1);
+       }
+
        /*
-        * Putting hundreds of bytes on the stack is considered rude.
-        * Static works because a tasklet can only be on one CPU at any time.
+        * This must occur at the end to ensure that we don't trash
+        * skb_shinfo until we're done.
         */
-       static u16 notify_list[NET_RX_RING_SIZE];
-
-       skb_queue_head_init(&rxq);
-
-       mcl = rx_mcl;
-       mmu = rx_mmu;
-       gop = grant_rx_op;
-
-       while ((skb = skb_dequeue(&rx_queue)) != NULL) {
-               netif   = netdev_priv(skb->dev);
-               vdata   = (unsigned long)skb->data;
-               old_mfn = virt_to_mfn(vdata);
-
-               if (!xen_feature(XENFEAT_auto_translated_physmap)) {
-                       /* Memory squeeze? Back off for an arbitrary while. */
-                       if ((new_mfn = alloc_mfn()) == 0) {
-                               if ( net_ratelimit() )
-                                       WPRINTK("Memory squeeze in netback "
-                                               "driver.\n");
-                               mod_timer(&net_timer, jiffies + HZ);
-                               skb_queue_head(&rx_queue, skb);
-                               break;
-                       }
-                       /*
-                        * Set the new P2M table entry before reassigning
-                        * the old data page. Heed the comment in
-                        * pgtable-2level.h:pte_page(). :-)
-                        */
-                       set_phys_to_machine(
-                               __pa(skb->data) >> PAGE_SHIFT,
-                               new_mfn);
-
-                       MULTI_update_va_mapping(mcl, vdata,
-                                               pfn_pte_ma(new_mfn,
-                                                          PAGE_KERNEL), 0);
-                       mcl++;
-
-                       mmu->ptr = ((maddr_t)new_mfn << PAGE_SHIFT) |
-                               MMU_MACHPHYS_UPDATE;
-                       mmu->val = __pa(vdata) >> PAGE_SHIFT;
-                       mmu++;
-               }
-
-               gop->mfn = old_mfn;
-               gop->domid = netif->domid;
-               gop->ref = RING_GET_REQUEST(
-                       &netif->rx, netif->rx.req_cons)->gref;
-               netif->rx.req_cons++;
-               gop++;
-
-               __skb_queue_tail(&rxq, skb);
-
-               /* Filled the batch queue? */
-               if ((gop - grant_rx_op) == ARRAY_SIZE(grant_rx_op))
-                       break;
-       }
-
-       if (!xen_feature(XENFEAT_auto_translated_physmap)) {
-               if (mcl == rx_mcl)
-                       return;
-
-               mcl[-1].args[MULTI_UVMFLAGS_INDEX] = UVMF_TLB_FLUSH|UVMF_ALL;
-
-               if (mmu - rx_mmu) {
-                       mcl->op = __HYPERVISOR_mmu_update;
-                       mcl->args[0] = (unsigned long)rx_mmu;
-                       mcl->args[1] = mmu - rx_mmu;
-                       mcl->args[2] = 0;
-                       mcl->args[3] = DOMID_SELF;
-                       mcl++;
-               }
-
-               ret = HYPERVISOR_multicall(rx_mcl, mcl - rx_mcl);
-               BUG_ON(ret != 0);
-       }
-
-       ret = HYPERVISOR_grant_table_op(GNTTABOP_transfer, grant_rx_op, 
-                                       gop - grant_rx_op);
-       BUG_ON(ret != 0);
-
-       mcl = rx_mcl;
-       gop = grant_rx_op;
-       while ((skb = __skb_dequeue(&rxq)) != NULL) {
-               netif   = netdev_priv(skb->dev);
-               size    = skb->tail - skb->data;
-
-               atomic_set(&(skb_shinfo(skb)->dataref), 1);
-               skb_shinfo(skb)->nr_frags = 0;
-               skb_shinfo(skb)->frag_list = NULL;
-
-               netif->stats.tx_bytes += size;
-               netif->stats.tx_packets++;
-
+       meta[count - nr_frags].id = netbk_gop_frag(netif,
+                                                  virt_to_page(skb->data),
+                                                  count - nr_frags, 0);
+       netif->rx.req_cons += nr_frags + 1;
+}
+
+static inline void netbk_free_pages(int nr_frags, struct netbk_rx_meta *meta)
+{
+       int i;
+
+       for (i = 0; i < nr_frags; i++)
+               put_page(meta[i].frag.page);
+}
+
+static int netbk_check_gop(int nr_frags, domid_t domid, int count)
+{
+       multicall_entry_t *mcl = rx_mcl + count;
+       gnttab_transfer_t *gop = grant_rx_op + count;
+       int status = NETIF_RSP_OKAY;
+       int i;
+
+       for (i = 0; i <= nr_frags; i++) {
                if (!xen_feature(XENFEAT_auto_translated_physmap)) {
                        /* The update_va_mapping() must not fail. */
                        BUG_ON(mcl->result != 0);
@@ -338,10 +380,9 @@ static void net_rx_action(unsigned long 
                }
 
                /* Check the reassignment error code. */
-               status = NETIF_RSP_OKAY;
                if (gop->status != 0) { 
                        DPRINTK("Bad status %d from grant transfer to DOM%u\n",
-                               gop->status, netif->domid);
+                               gop->status, domid);
                        /*
                         * Page no longer belongs to us unless GNTST_bad_page,
                         * but that should be a fatal error anyway.
@@ -349,17 +390,128 @@ static void net_rx_action(unsigned long 
                        BUG_ON(gop->status == GNTST_bad_page);
                        status = NETIF_RSP_ERROR; 
                }
-               irq = netif->irq;
-               id = RING_GET_REQUEST(&netif->rx, netif->rx.rsp_prod_pvt)->id;
-               flags = 0;
+               gop++;
+       }
+
+       return status;
+}
+
+static void netbk_add_frag_responses(netif_t *netif, int status,
+                                    struct netbk_rx_meta *meta, int nr_frags)
+{
+       int i;
+
+       for (i = 0; i < nr_frags; i++) {
+               int id = meta[i].id;
+               int flags = (i == nr_frags - 1) ? 0 : NETRXF_more_data;
+
+               make_rx_response(netif, id, status, meta[i].frag.page_offset,
+                                meta[i].frag.size, flags);
+       }
+}
+
+static void net_rx_action(unsigned long unused)
+{
+       netif_t *netif = NULL; 
+       s8 status;
+       u16 id, irq, flags;
+       multicall_entry_t *mcl;
+       struct sk_buff_head rxq;
+       struct sk_buff *skb;
+       int notify_nr = 0;
+       int ret;
+       int nr_frags;
+       int count;
+
+       /*
+        * Putting hundreds of bytes on the stack is considered rude.
+        * Static works because a tasklet can only be on one CPU at any time.
+        */
+       static u16 notify_list[NET_RX_RING_SIZE];
+       static struct netbk_rx_meta meta[NET_RX_RING_SIZE];
+
+       skb_queue_head_init(&rxq);
+
+       count = 0;
+
+       while ((skb = skb_dequeue(&rx_queue)) != NULL) {
+               nr_frags = skb_shinfo(skb)->nr_frags;
+               *(int *)skb->cb = nr_frags;
+
+               if (!xen_feature(XENFEAT_auto_translated_physmap) &&
+                   check_mfn(nr_frags + 1)) {
+                       /* Memory squeeze? Back off for an arbitrary while. */
+                       if ( net_ratelimit() )
+                               WPRINTK("Memory squeeze in netback "
+                                       "driver.\n");
+                       mod_timer(&net_timer, jiffies + HZ);
+                       skb_queue_head(&rx_queue, skb);
+                       break;
+               }
+
+               netbk_gop_skb(skb, meta, count);
+
+               count += nr_frags + 1;
+
+               __skb_queue_tail(&rxq, skb);
+
+               /* Filled the batch queue? */
+               if (count + MAX_SKB_FRAGS >= NET_RX_RING_SIZE)
+                       break;
+       }
+
+       if (!count)
+               return;
+
+       if (!xen_feature(XENFEAT_auto_translated_physmap)) {
+               mcl = rx_mcl + count;
+
+               mcl[-1].args[MULTI_UVMFLAGS_INDEX] = UVMF_TLB_FLUSH|UVMF_ALL;
+
+               mcl->op = __HYPERVISOR_mmu_update;
+               mcl->args[0] = (unsigned long)rx_mmu;
+               mcl->args[1] = count;
+               mcl->args[2] = 0;
+               mcl->args[3] = DOMID_SELF;
+
+               ret = HYPERVISOR_multicall(rx_mcl, count + 1);
+               BUG_ON(ret != 0);
+       }
+
+       ret = HYPERVISOR_grant_table_op(GNTTABOP_transfer, grant_rx_op, count);
+       BUG_ON(ret != 0);
+
+       count = 0;
+       while ((skb = __skb_dequeue(&rxq)) != NULL) {
+               nr_frags = *(int *)skb->cb;
+
+               atomic_set(&(skb_shinfo(skb)->dataref), 1);
+               skb_shinfo(skb)->nr_frags = 0;
+               skb_shinfo(skb)->frag_list = NULL;
+
+               netif = netdev_priv(skb->dev);
+               netif->stats.tx_bytes += skb->len;
+               netif->stats.tx_packets++;
+
+               netbk_free_pages(nr_frags, meta + count + 1);
+               status = netbk_check_gop(nr_frags, netif->domid, count);
+
+               id = meta[count].id;
+               flags = nr_frags ? NETRXF_more_data : 0;
+
                if (skb->ip_summed == CHECKSUM_HW) /* local packet? */
                        flags |= NETRXF_csum_blank | NETRXF_data_validated;
                else if (skb->proto_data_valid) /* remote but checksummed? */
                        flags |= NETRXF_data_validated;
-               if (make_rx_response(netif, id, status,
-                                    (unsigned long)skb->data & ~PAGE_MASK,
-                                    size, flags) &&
-                   (rx_notify[irq] == 0)) {
+
+               make_rx_response(netif, id, status, offset_in_page(skb->data),
+                                skb_headlen(skb), flags);
+               netbk_add_frag_responses(netif, status, meta + count + 1,
+                                        nr_frags);
+
+               RING_PUSH_RESPONSES_AND_CHECK_NOTIFY(&netif->rx, ret);
+               irq = netif->irq;
+               if (ret && !rx_notify[irq]) {
                        rx_notify[irq] = 1;
                        notify_list[notify_nr++] = irq;
                }
@@ -370,7 +522,7 @@ static void net_rx_action(unsigned long 
 
                netif_put(netif);
                dev_kfree_skb(skb);
-               gop++;
+               count += nr_frags + 1;
        }
 
        while (notify_nr != 0) {
@@ -1039,7 +1191,6 @@ static int make_rx_response(netif_t *net
 {
        RING_IDX i = netif->rx.rsp_prod_pvt;
        netif_rx_response_t *resp;
-       int notify;
 
        resp = RING_GET_RESPONSE(&netif->rx, i);
        resp->offset     = offset;
@@ -1050,9 +1201,8 @@ static int make_rx_response(netif_t *net
                resp->status = (s16)st;
 
        netif->rx.rsp_prod_pvt = ++i;
-       RING_PUSH_RESPONSES_AND_CHECK_NOTIFY(&netif->rx, notify);
-
-       return notify;
+
+       return 0;
 }
 
 #ifdef NETBE_DEBUG_INTERRUPT
diff -r c82398c08669 -r 54f84d198a1d 
linux-2.6-xen-sparse/drivers/xen/netback/xenbus.c
--- a/linux-2.6-xen-sparse/drivers/xen/netback/xenbus.c Fri Jul 07 23:37:45 
2006 +1000
+++ b/linux-2.6-xen-sparse/drivers/xen/netback/xenbus.c Fri Jul 07 23:38:49 
2006 +1000
@@ -377,6 +377,13 @@ static int connect_rings(struct backend_
                /* Must be non-zero for pfifo_fast to work. */
                be->netif->dev->tx_queue_len = 1;
 
+       if (xenbus_scanf(XBT_NIL, dev->otherend, "feature-sg", "%d", &val) < 0)
+               val = 0;
+       if (val) {
+               be->netif->features |= NETIF_F_SG;
+               be->netif->dev->features |= NETIF_F_SG;
+       }
+
        /* Map the shared frame, irq etc. */
        err = netif_map(be->netif, tx_ring_ref, rx_ring_ref, evtchn);
        if (err) {

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