xen-netback: coalesce slots in TX path and fix regressions

[~andy/linux] / include / xen / interface / io / netif.h

diff --git a/include/xen/interface/io/netif.h b/include/xen/interface/io/netif.h
index 58fadcac33a17c004c1cdf17557bc570b8a4f33e..3ef3fe05ee99c55aa27d7c3eb5f828935b89ed59 100644 (file)
--- a/include/xen/interface/io/netif.h
+++ b/include/xen/interface/io/netif.h
@@ -12,6 +12,24 @@
 #include <xen/interface/io/ring.h>
 #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.