xen: Add support for preemption

Add Xen support for preemption.  This is mostly a cleanup of existing
preempt_enable/disable calls, or just comments to explain the current
usage.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>

1 files changed, 17 insertions, 5 deletions

diff --git a/arch/i386/xen/time.c b/arch/i386/xen/time.c
index aeb04cf5dbf1..51fdabf1fd4d 100644
--- a/arch/i386/xen/time.c
+++ b/arch/i386/xen/time.c
@@ -88,7 +88,7 @@ static void get_runstate_snapshot(struct vcpu_runstate_info *res)
         u64 state_time;
         struct vcpu_runstate_info *state;
 
-        preempt_disable();
+        BUG_ON(preemptible());
 
         state = &__get_cpu_var(runstate);
 
@@ -103,8 +103,6 @@ static void get_runstate_snapshot(struct vcpu_runstate_info *res)
                 *res = *state;
                 barrier();
         } while (get64(&state->state_entry_time) != state_time);
-
-        preempt_enable();
 }
 
 static void setup_runstate_info(int cpu)
@@ -179,9 +177,19 @@ static void do_stolen_accounting(void)
 unsigned long long xen_sched_clock(void)
 {
         struct vcpu_runstate_info state;
-        cycle_t now = xen_clocksource_read();
+        cycle_t now;
+        u64 ret;
         s64 offset;
 
+        /*
+         * Ideally sched_clock should be called on a per-cpu basis
+         * anyway, so preempt should already be disabled, but that's
+         * not current practice at the moment.
+         */
+        preempt_disable();
+
+        now = xen_clocksource_read();
+
         get_runstate_snapshot(&state);
 
         WARN_ON(state.state != RUNSTATE_running);
@@ -190,9 +198,13 @@ unsigned long long xen_sched_clock(void)
         if (offset < 0)
                 offset = 0;
 
-        return state.time[RUNSTATE_blocked] +
+        ret = state.time[RUNSTATE_blocked] +
                 state.time[RUNSTATE_running] +
                 offset;
+
+        preempt_enable();
+
+        return ret;
 }