* Thomas Garnier [off-list ref] wrote:

This patch makes the GDT remapped pages read-only to prevent corruption.
This change is done only on 64 bit.

Please spell '64-bit' consistently through the series. I've seen two variants:

  64 bit
  64bit

+/*
+ * The LTR instruction marks the TSS GDT entry as busy. In 64bit, the GDT is
+ * a read-only remapping. To prevent a page fault, the GDT is switched to the
+ * original writeable version when needed.

s/In 64bit,
 /On 64-bit kernels,

+ */
+#ifdef CONFIG_X86_64
+static inline void native_load_tr_desc(void)
+{
+	struct desc_ptr gdt;
+	int cpu = raw_smp_processor_id();
+	bool restore = false;
+	struct desc_struct *fixmap_gdt;
+
+	native_store_gdt(&gdt);
+	fixmap_gdt = get_cpu_fixmap_gdt(cpu);
+
+	/*
+	 * If the current GDT is the read-only fixmap, swap to the original
+	 * writeable version. Swap back at the end.
+	 */
+	if (gdt.address == (unsigned long)fixmap_gdt) {
+		load_direct_gdt(cpu);
+		restore = true;
+	}
+	asm volatile("ltr %w0"::"q" (GDT_ENTRY_TSS*8));
+	if (restore)
+		load_fixmap_gdt(cpu);

Please use bool plus 0/1, it's more readable (to me) than the true/false notation.

 extern void switch_to_new_gdt(int);
+extern void load_direct_gdt(int);
 extern void load_fixmap_gdt(int);

+/* Load the original GDT from the per-cpu structure */
+void load_direct_gdt(int cpu)
+{
+	struct desc_ptr gdt_descr;
+
+	gdt_descr.address = (long)get_cpu_direct_gdt(cpu);

Please name the functions in an easier to understand way, such as:

	get_cpu_gdt_rw()
	get_cpu_gdt_ro()

that the GDT is in the direct mappings is less important than the fact the the 
address is writable ...

+}
+EXPORT_SYMBOL(load_direct_gdt);

EXPORT_SYMBOL_GPL(), or no export at all.

+EXPORT_SYMBOL(load_fixmap_gdt);

ditto.

 	 * VT restores TR but not its size.  Useless.
 	 */
-	struct desc_ptr *gdt = this_cpu_ptr(&host_gdt);
 	struct desc_struct *descs;
 
-	descs = (void *)gdt->address;
+	descs = (void *)get_current_direct_gdt();

Couldn't the type cast be dropped?

 
-	table_base = gdt->address;
+	table_base = (unsigned long)get_current_direct_gdt();

Instead of spreading these type casts far and wide please introduce another 
accessor the returns 'unsigned long':

	get_cpu_gdt_rw_vaddr()

or such.

Thanks,

	Ingo