x86: re-enable VCPUOP_register_vcpu_time_memory_area

By moving the call to update_vcpu_system_time() out of schedule() into
arch-specific context switch code, the original problem of the function
accessing the wrong domain's address space goes away (obvious even from
patch context, as update_runstate_area() does similar copying).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Regardless of the code freeze I'd still like to propose this for
inclusion in 4.3, mainly based on the fact that this got disabled late
in the 4.0 release cycle with the expectation that it would get
re-enabled soon after. Now that upstream Linux also has, as of 3.8 at
least on x86-64, the necessary hypervisor independent support code, it
would be odd to not leverage this on Xen.

--- a/xen/arch/arm/domain.c
+++ b/xen/arch/arm/domain.c
@@ -232,6 +232,9 @@ static void schedule_tail(struct vcpu *p
 
     if ( prev != current )
         update_runstate_area(current);
+
+    /* Ensure that the vcpu has an up-to-date time base. */
+    update_vcpu_system_time(current);
 }
 
 static void continue_new_vcpu(struct vcpu *prev)
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -966,11 +966,6 @@ arch_do_vcpu_op(
 
     switch ( cmd )
     {
-    /*
-     * XXX Disable for 4.0.0: __update_vcpu_system_time() writes to the given
-     * virtual address even when running in another domain's address space.
-     */
-#if 0
     case VCPUOP_register_vcpu_time_memory_area:
     {
         struct vcpu_register_time_memory_area area;
@@ -989,7 +984,6 @@ arch_do_vcpu_op(
 
         break;
     }
-#endif
 
     case VCPUOP_get_physid:
     {
@@ -1457,6 +1451,9 @@ void context_switch(struct vcpu *prev, s
     if (prev != next)
         update_runstate_area(next);
 
+    /* Ensure that the vcpu has an up-to-date time base. */
+    update_vcpu_system_time(next);
+
     schedule_tail(next);
     BUG();
 }
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -3174,6 +3174,7 @@ static long hvm_vcpu_op(
     case VCPUOP_set_singleshot_timer:
     case VCPUOP_stop_singleshot_timer:
     case VCPUOP_register_vcpu_info:
+    case VCPUOP_register_vcpu_time_memory_area:
         rc = do_vcpu_op(cmd, vcpuid, arg);
         break;
     default:
@@ -3232,6 +3233,7 @@ static long hvm_vcpu_op_compat32(
     case VCPUOP_set_singleshot_timer:
     case VCPUOP_stop_singleshot_timer:
     case VCPUOP_register_vcpu_info:
+    case VCPUOP_register_vcpu_time_memory_area:
         rc = compat_vcpu_op(cmd, vcpuid, arg);
         break;
     default:
--- a/xen/common/schedule.c
+++ b/xen/common/schedule.c
@@ -1231,8 +1231,6 @@ static void schedule(void)
     if ( next_slice.migrated )
         evtchn_move_pirqs(next);
 
-    /* Ensure that the domain has an up-to-date time base. */
-    update_vcpu_system_time(next);
     vcpu_periodic_timer_work(next);
 
     context_switch(prev, next);