x86/xen/gdt: Use X86_FEATURE_XENPV instead of globals for the GDT fixup

Xen imposes special requirements on the GDT.  Rather than using a
global variable for the pgprot, just use an explicit special case
for Xen -- this makes it clearer what's going on.  It also debloats
64-bit kernels very slightly.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Garnier <thgarnie@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/e9ea96abbfd6a8c87753849171bb5987ecfeb523.1490218061.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/desc.h

@@ -39,7 +39,6 @@ extern struct desc_ptr idt_descr;
 extern gate_desc idt_table[];
 extern const struct desc_ptr debug_idt_descr;
 extern gate_desc debug_idt_table[];
-extern pgprot_t pg_fixmap_gdt_flags;
 
 struct gdt_page {
 	struct desc_struct gdt[GDT_ENTRIES];

arch/x86/kernel/cpu/common.c

@@ -448,21 +448,27 @@ void load_percpu_segment(int cpu)
 	load_stack_canary_segment();
 }
 
-/*
- * On 64-bit the GDT remapping is read-only.
- * A global is used for Xen to change the default when required.
- */
-#ifdef CONFIG_X86_64
-pgprot_t pg_fixmap_gdt_flags = PAGE_KERNEL_RO;
-#else
-pgprot_t pg_fixmap_gdt_flags = PAGE_KERNEL;
-#endif
-
 /* Setup the fixmap mapping only once per-processor */
 static inline void setup_fixmap_gdt(int cpu)
 {
-	__set_fixmap(get_cpu_gdt_ro_index(cpu), get_cpu_gdt_paddr(cpu),
-		     pg_fixmap_gdt_flags);
+#ifdef CONFIG_X86_64
+	/* On 64-bit systems, we use a read-only fixmap GDT. */
+	pgprot_t prot = PAGE_KERNEL_RO;
+#else
+	/*
+	 * On native 32-bit systems, the GDT cannot be read-only because
+	 * our double fault handler uses a task gate, and entering through
+	 * a task gate needs to change an available TSS to busy.  If the GDT
+	 * is read-only, that will triple fault.
+	 *
+	 * On Xen PV, the GDT must be read-only because the hypervisor requires
+	 * it.
+	 */
+	pgprot_t prot = boot_cpu_has(X86_FEATURE_XENPV) ?
+		PAGE_KERNEL_RO : PAGE_KERNEL;
+#endif
+
+	__set_fixmap(get_cpu_gdt_ro_index(cpu), get_cpu_gdt_paddr(cpu), prot);
 }
 
 /* Load the original GDT from the per-cpu structure */

arch/x86/xen/enlighten.c

@@ -1545,9 +1545,6 @@ asmlinkage __visible void __init xen_start_kernel(void)
 	 */
 	xen_initial_gdt = &per_cpu(gdt_page, 0);
 
-	/* GDT can only be remapped RO */
-	pg_fixmap_gdt_flags = PAGE_KERNEL_RO;
-
 	xen_smp_init();
 
 #ifdef CONFIG_ACPI_NUMA