Changeset 52815 in vbox

Timestamp:

Sep 22, 2014 10:33:36 AM (10 years ago)

Author:

vboxsync

svn:sync-xref-src-repo-rev:

96211

Message:

HostDrivers/Support: Comments and use a proper error code for failing to measure deltas.

Location:

trunk

Files:

• include/VBox/err.h (modified) (1 diff)
• src/VBox/HostDrivers/Support/SUPDrv.c (modified) (4 diffs)

trunk/include/VBox/err.h

@@ -1767 +1767 @@
 /** Type error opening the ApiPort LPC object. */
 #define VERR_SUPDRV_APIPORT_OPEN_ERROR_TYPE         (-3742)
+/** Failed to measure the TSC delta between two CPUs. */
+#define VERR_SUPDRV_TSC_DELTA_MEASUREMENT_FAILED    (-3743)
 /** @} */
 

trunk/src/VBox/HostDrivers/Support/SUPDrv.c

@@ -6074 +6074 @@
  * @remarks Measuring TSC deltas between the CPUs is tricky because we need to
  *     read the TSC at exactly the same time on both the master and the worker
- *     CPUs. Due to DMA, bus arbitration, cache locality, contention etc. there
- *     is no guaranteed way of doing this on x86 CPUs. We try to minimize the
- *     measurement error by computing the minimum read time of the compare
- *     statement in the worker by taking TSC measurements across it. We also
- *     ignore the first few runs of the loop in order to prime the cache.
+ *     CPUs. Due to DMA, bus arbitration, cache locality, contention, pipelining
+ *     etc. there is no guaranteed way of doing this on x86 CPUs. We try to
+ *     minimize the measurement error by computing the minimum read time of the
+ *     compare statement in the worker by taking TSC measurements across it.
+ *
+ *     We ignore the first few runs of the loop in order to prime the cache.
+ *     Also, be careful about using 'pause' instruction in critical busy-wait
+ *     loops in this code - it can cause undesired behaviour with
+ *     hyperthreading.
  *
  *     It must be noted that the computed minimum read time is mostly to
@@ -6087 +6091 @@
  *     earlier. As long as these occasional negative deltas are lower than the
  *     time it takes to exit guest-context and the OS to reschedule EMT on a
- *     different CPU we won't expose a TSC that jumped backwards.
+ *     different CPU we won't expose a TSC that jumped backwards. It is because
+ *     of the existence of the negative deltas we don't recompute the delta with
+ *     the master and worker interchanged to eliminate the remaining measurement
+ *     error.
  */
 static DECLCALLBACK(void) supdrvMeasureTscDeltaCallback(RTCPUID idCpu, void *pvUser1, void *pvUser2)
@@ -6304 +6311 @@
     {
         /* Fire TSC-read workers on all CPUs but only synchronize between master and one worker to ease memory contention. */
+        ASMAtomicWriteS64(&pGipCpuWorker->i64TSCDelta, INT64_MAX);
         ASMAtomicWriteU32(&g_pTscDeltaSync->u, GIP_TSC_DELTA_SYNC_STOP);
         rc = RTMpOnAll(supdrvMeasureTscDeltaCallback, pGip, &pGipCpuWorker->idCpu);
@@ -6309 +6317 @@
         {
             if (RT_UNLIKELY(pGipCpuWorker->i64TSCDelta == INT64_MAX))
-                rc = VERR_UNRESOLVED_ERROR;
+                rc = VERR_SUPDRV_TSC_DELTA_MEASUREMENT_FAILED;
         }
     }