Re: [Xen-devel] [PATCH 3/8] x86/time: introduce and use rdtsc_ordered()

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 20.06.16 at 14:59, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 15/06/16 11:27, Jan Beulich wrote:
>> Matching Linux commit 03b9730b76 ("x86/asm/tsc: Add rdtsc_ordered() and
>> use it in trivial call sites") and earlier ones it builds upon, let's
>> make sure timing loops don't have their rdtsc()-s re-ordered, as that
>> would harm precision of the result (values were observed to be several
>> hundred clocks off without this adjustment).
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

I have these two additional hunks for v2:

@@ -1124,16 +1124,13 @@ static void local_time_calibration(void)
  */
 static void check_tsc_warp(unsigned long tsc_khz, unsigned long *max_warp)
 {
-#define rdtsc_barrier() mb()
     static DEFINE_SPINLOCK(sync_lock);
     static cycles_t last_tsc;
 
     cycles_t start, now, prev, end;
     int i;
 
-    rdtsc_barrier();
-    start = get_cycles();
-    rdtsc_barrier();
+    start = rdtsc_ordered();
 
     /* The measurement runs for 20 msecs: */
     end = start + tsc_khz * 20ULL;
@@ -1148,9 +1145,7 @@ static void check_tsc_warp(unsigned long
          */
         spin_lock(&sync_lock);
         prev = last_tsc;
-        rdtsc_barrier();
-        now = get_cycles();
-        rdtsc_barrier();
+        now = rdtsc_ordered();
         last_tsc = now;
         spin_unlock(&sync_lock);
 

May I consider those covered as well?

Jan


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