diff --git a/sys/man/8/plan9.ini b/sys/man/8/plan9.ini
index 61115f7fd..48aab7f37 100644
--- a/sys/man/8/plan9.ini
+++ b/sys/man/8/plan9.ini
@@ -720,6 +720,11 @@ processors.
 Prints a summary of the multiprocessor APIC interrupt configuration.
 .SS \fL*nomsi=\fP
 Disables message signaled interrupts.
+.SS \fL*notsc=\fP
+Disables the use of the per processor timestamp counter registers
+as high resolution clock. This option is intended for machines where
+the timestamp counter rate is not constant across cores or is adjusted
+dynamically by power management.
 .SS \fL*pcimaxbno=value\fP
 This puts a limit on the maximum bus number probed
 on a PCI bus (default 7).
diff --git a/sys/src/9/pc/devarch.c b/sys/src/9/pc/devarch.c
index be29eab3c..90cc2f4b9 100644
--- a/sys/src/9/pc/devarch.c
+++ b/sys/src/9/pc/devarch.c
@@ -797,7 +797,7 @@ cpuidentify(void)
 	/*
 	 *  if there is one, set tsc to a known value
 	 */
-	if(m->cpuiddx & Tsc){
+	if((m->cpuiddx & Tsc) != 0 && getconf("*notsc") == nil){
 		m->havetsc = 1;
 		cycles = _cycles;
 		if(m->cpuiddx & Cpumsr)