+static void detect_null_seg_behavior(struct cpuinfo_x86 *c)
+{
+#ifdef CONFIG_X86_64
+ /*
+ * Empirically, writing zero to a segment selector on AMD does
+ * not clear the base, whereas writing zero to a segment
+ * selector on Intel does clear the base. Intel's behavior
+ * allows slightly faster context switches in the common case
+ * where GS is unused by the prev and next threads.
+ *
+ * Since neither vendor documents this anywhere that I can see,
+ * detect it directly instead of hardcoding the choice by
+ * vendor.
+ *
+ * I've designated AMD's behavior as the "bug" because it's
+ * counterintuitive and less friendly.
+ */
+
+ unsigned long old_base, tmp;
+ rdmsrl(MSR_FS_BASE, old_base);
+ wrmsrl(MSR_FS_BASE, 1);
+ loadsegment(fs, 0);
+ rdmsrl(MSR_FS_BASE, tmp);
+ if (tmp != 0)
+ set_cpu_bug(c, X86_BUG_NULL_SEG);
+ wrmsrl(MSR_FS_BASE, old_base);
+#endif
+}
+