[Xen-changelog] [xen master] netif: define XEN_NETIF_NR_SLOTS_MIN in public header

[patchbot@xxxxxxx]

commit 723a375f4e283f2d2b730e98cf6f6cc226423a7f
Author:     Wei Liu <wei.liu2@xxxxxxxxxx>
AuthorDate: Tue May 7 16:54:56 2013 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue May 7 16:54:56 2013 +0200

    netif: define XEN_NETIF_NR_SLOTS_MIN in public header
    
    Xen network protocol has implicit dependency on MAX_SKB_FRAGS. In order to
    remove dependency on MAX_SKB_FRAGS, we derive a constant from historical
    MAX_SKB_FRAGS for future reference.
    
    Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>
    Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
    Acked-by: Keir Fraser <keir@xxxxxxx>
---
 xen/include/public/io/netif.h |   18 ++++++++++++++++++
 1 files changed, 18 insertions(+), 0 deletions(-)

diff --git a/xen/include/public/io/netif.h b/xen/include/public/io/netif.h
index ccde53b..9664998 100644
--- a/xen/include/public/io/netif.h
+++ b/xen/include/public/io/netif.h
@@ -31,6 +31,24 @@
 #include "../grant_table.h"
 
 /*
+ * Older implementation of Xen network frontend / backend has an
+ * implicit dependency on the MAX_SKB_FRAGS as the maximum number of
+ * ring slots a skb can use. Netfront / netback may not work as
+ * expected when frontend and backend have different MAX_SKB_FRAGS.
+ *
+ * A better approach is to add mechanism for netfront / netback to
+ * negotiate this value. However we cannot fix all possible
+ * frontends, so we need to define a value which states the minimum
+ * slots backend must support.
+ *
+ * The minimum value derives from older Linux kernel's MAX_SKB_FRAGS
+ * (18), which is proved to work with most frontends. Any new backend
+ * which doesn't negotiate with frontend should expect frontend to
+ * send a valid packet using slots up to this value.
+ */
+#define XEN_NETIF_NR_SLOTS_MIN 18
+
+/*
  * Notifications after enqueuing any type of message should be conditional on
  * the appropriate req_event or rsp_event field in the shared ring.
  * If the client sends notification for rx requests then it should specify
--
generated by git-patchbot for /home/xen/git/xen.git#master

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