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>

1 files changed, 22 insertions, 9 deletions

diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 0172ba77452..c13698faae5 100644
--- a/arch/x86/xen/enlighten.c
+++ b/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