xen64: Clear %fs on xen_load_tls()

We need to do this, otherwise we can get a GPF on hypercall return
after TLS descriptor is cleared but %fs is still pointing to it.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Stephen Tweedie <sct@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/xen/enlighten.c

@@ -364,14 +364,6 @@ static void load_TLS_descriptor(struct thread_struct *t,
 
 static void xen_load_tls(struct thread_struct *t, unsigned int cpu)
 {
-	xen_mc_batch();
-
-	load_TLS_descriptor(t, cpu, 0);
-	load_TLS_descriptor(t, cpu, 1);
-	load_TLS_descriptor(t, cpu, 2);
-
-	xen_mc_issue(PARAVIRT_LAZY_CPU);
-
 	/*
 	 * XXX sleazy hack: If we're being called in a lazy-cpu zone,
 	 * it means we're in a context switch, and %gs has just been
@@ -380,9 +372,30 @@ static void xen_load_tls(struct thread_struct *t, unsigned int cpu)
 	 * Either way, it has been saved, and the new value will get
 	 * loaded properly.  This will go away as soon as Xen has been
 	 * modified to not save/restore %gs for normal hypercalls.
+	 *
+	 * On x86_64, this hack is not used for %gs, because gs points
+	 * to KERNEL_GS_BASE (and uses it for PDA references), so we
+	 * must not zero %gs on x86_64
+	 *
+	 * For x86_64, we need to zero %fs, otherwise we may get an
+	 * exception between the new %fs descriptor being loaded and
+	 * %fs being effectively cleared at __switch_to().
 	 */
-	if (paravirt_get_lazy_mode() == PARAVIRT_LAZY_CPU)
+	if (paravirt_get_lazy_mode() == PARAVIRT_LAZY_CPU) {
+#ifdef CONFIG_X86_32
 		loadsegment(gs, 0);
+#else
+		loadsegment(fs, 0);
+#endif
+	}
+
+	xen_mc_batch();
+
+	load_TLS_descriptor(t, cpu, 0);
+	load_TLS_descriptor(t, cpu, 1);
+	load_TLS_descriptor(t, cpu, 2);
+
+	xen_mc_issue(PARAVIRT_LAZY_CPU);
 }
 
 #ifdef CONFIG_X86_64