[PATCH V3 10/10] perf intel-pt: Add documentation for new clock IDs

[Adrian Hunter]

Add brief documentation for new clock IDs CLOCK_PERF_HW_CLOCK and
CLOCK_PERF_HW_CLOCK_NS, as well as new config variables
intel-pt.max_nonturbo_ratio and intel-pt.tsc_art_ratio.

Signed-off-by: Adrian Hunter <adrian.hunter@xxxxxxxxx>
---
 tools/perf/Documentation/perf-intel-pt.txt | 45 ++++++++++++++++++++++
 1 file changed, 45 insertions(+)

diff --git a/tools/perf/Documentation/perf-intel-pt.txt b/tools/perf/Documentation/perf-intel-pt.txt
index ff58bd4c381b..383126d05577 100644
--- a/tools/perf/Documentation/perf-intel-pt.txt
+++ b/tools/perf/Documentation/perf-intel-pt.txt
@@ -509,6 +509,29 @@ notnt		Disable TNT packets.  Without TNT packets, it is not possible to walk
 		"0" otherwise.
 
 
+perf event clock
+~~~~~~~~~~~~~~~~
+
+Newer kernel and tools support 2 special clocks: CLOCK_PERF_HW_CLOCK which is
+TSC and CLOCK_PERF_HW_CLOCK_NS which is TSC converted to nanoseconds.
+CLOCK_PERF_HW_CLOCK_NS is the same as the default perf event clock, but it is
+not subject to paravirtualization, so it still works with Intel PT in a VM
+guest.  Note however, TSC may not be being fully virtualized, so the results
+may be unexpected.
+
+	--clockid CLOCK_PERF_HW_CLOCK_NS
+
+To use TSC instead of nanoseconds, use the option:
+
+	--clockid CLOCK_PERF_HW_CLOCK
+
+Beware forgetting that the time stamp of events will show TSC ticks
+(divided by 1,000,000,000) not seconds.
+
+Other clocks are not supported for use with Intel PT because they cannot be
+converted to/from TSC.
+
+
 AUX area sampling option
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -1398,6 +1421,28 @@ There were none.
           :17006 17006 [001] 11500.262869216:  ffffffff8220116e error_entry+0xe ([guest.kernel.kallsyms])               pushq  %rax
 
 
+Tracing within a Virtual Machine
+--------------------------------
+
+When supported, using Intel PT within a virtual machine does not support TSC
+because the perf event clock is subject to paravirtualization.  That is
+overcome by the new CLOCK_PERF_HW_CLOCK_NS clock - refer 'perf event clock'
+above.  In addition, in a VM, the following might be zero:
+
+	/sys/bus/event_source/devices/intel_pt/max_nonturbo_ratio
+	/sys/bus/event_source/devices/intel_pt/tsc_art_ratio
+
+The decoder needs this information to correctly interpret timing packets,
+so the values can be provided by config variables in that case. Note in
+the absence of VMCS TSC Scaling, this is probably the same as the host values.
+The config variables are:
+
+	intel-pt.max_nonturbo_ratio
+	intel-pt.tsc_art_ratio
+
+For more information about perf config variables, refer linkperf:perf-config[1]
+
+
 Event Trace
 -----------
 
-- 
2.25.1