+ * We leave smp_rmb/smp_wmb as full barriers for processors that do not have
+ * fence instructions.
+ *
+ * An empty cmm_smp_rmb() may not be enough on old PentiumPro multiprocessor
+ * systems, due to an erratum. The Linux kernel says that "Even distro
+ * kernels should think twice before enabling this", but for now let's
+ * be conservative and leave the full barrier on 32-bit processors. Also,
+ * IDT WinChip supports weak store ordering, and the kernel may enable it
+ * under our feet; cmm_smp_wmb() ceases to be a nop for these processors.