3. Enable two-buffer mode(for Rx path) automatically for SGI
   systems. This improves Rx performance dramatically on 
   SGI systems.

+/* Enable 2 buffer mode by default for SGI system */
+#ifdef CONFIG_IA64_SGI_SN2
+#define CONFIG_2BUFF_MODE
+#endif

this enabled it only on kernel that are built to only run on SN2
hardware, which is completely useless in practice.  Besides that defining
a CONFIG_ symbol from source files is a big no-go.

What exactly is the 2buff mode, and more specific what are the downsides
of  enabling it on non-SGI hardware?