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-rw-r--r--include/xen/interface/io/netif.h18
1 files changed, 18 insertions, 0 deletions
diff --git a/include/xen/interface/io/netif.h b/include/xen/interface/io/netif.h
index 9dfc1200098..e829a0922e1 100644
--- a/include/xen/interface/io/netif.h
+++ b/include/xen/interface/io/netif.h
@@ -13,6 +13,24 @@
#include <xen/interface/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