[Xen-devel] [PATCH] x86/time: use correct (local) time stamp in constant-TSC calibration fast path

["Jan Beulich" <JBeulich@xxxxxxxx>]

This looks like a copy and paste mistake in commit 1b6a99892d ("x86:
Simpler time handling when TSC is constant across all power saving
states"), responsible for occasional many-microsecond cross-CPU skew of
what NOW() returns.

Also improve the correlation between local TSC and stime stamps
obtained at the end of the two calibration handlers: Compute the stime
one from the TSC one, instead of doing another rdtsc() for that
compuation.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
As to 4.7 inclusion: This of course looks like a pretty blatant mistake
that has been there for many years (and hence many releases). There's
certainly non-zero risk that I'm overlooking something here (despite
Joao apparently having come to the same conclusion), so I can't really
make up my mind on whether to request this patch to be put there right
away, or rather having linger in -unstable for a while.

--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -998,7 +998,7 @@ static void local_time_calibration(void)
         /* Atomically read cpu_calibration struct and write cpu_time struct. */
         local_irq_disable();
         t->local_tsc_stamp    = c->local_tsc_stamp;
-        t->stime_local_stamp  = c->stime_master_stamp;
+        t->stime_local_stamp  = c->stime_local_stamp;
         t->stime_master_stamp = c->stime_master_stamp;
         local_irq_enable();
         update_vcpu_system_time(current);
@@ -1275,7 +1275,7 @@ static void time_calibration_tsc_rendezv
     }
 
     c->local_tsc_stamp = rdtsc();
-    c->stime_local_stamp = get_s_time();
+    c->stime_local_stamp = get_s_time_fixed(c->local_tsc_stamp);
     c->stime_master_stamp = r->master_stime;
 
     raise_softirq(TIME_CALIBRATE_SOFTIRQ);
@@ -1305,7 +1305,7 @@ static void time_calibration_std_rendezv
     }
 
     c->local_tsc_stamp = rdtsc();
-    c->stime_local_stamp = get_s_time();
+    c->stime_local_stamp = get_s_time_fixed(c->local_tsc_stamp);
     c->stime_master_stamp = r->master_stime;
 
     raise_softirq(TIME_CALIBRATE_SOFTIRQ);

Attachment: x86-time-use-local-stamp.patch
Description: Text document

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