Re: [PATCH 0/3] Improve TSC as a clocksource under VMware

From: Alok Kataria
Date: Wed Oct 22 2008 - 15:26:39 EST

Jeff, i have kept your Signed-off-by with this slightly modified patch,
incase you have any objections to that please let me know.

[X86] Skip verification by the watchdog for TSC clocksource.

From: Alok N Kataria <akataria@xxxxxxxxxx>

This is achieved by resetting the CLOCKSOURCE_MUST_VERIFY flag.

We add a tsc=reliable commandline option to enable this.
This enables legacy hardware without HPET, LAPIC, or ACPI timers
to enter high-resolution timer mode.

Along with that have extended this to be used in virtualization
environment, just for VMware as yet. This is important since there
is a wrap-around problem with the acpi_pm timer.
The acpi_pm counter is just 24bits and this can overflow in 4 seconds.
With the NO_HZ kernels in virtualized environment, there can be situations
when the guest is descheduled for longer duration, as a result we may miss
the wrap of the acpi counter. When TSC is used as a clocksource and acpi_pm
timer is being used as the watchdog clocksource this error in acpi_pm
results in TSC being marked as unstable, and essentially results in time
dropping in chunks of 4 seconds whenever this wrap is missed. Since the
virtualized TSC is reliable on VMware, we should always use the TSCs
clocksource on VMware, so we skip the verfication at runtime.

Since we reset the flag for mgeode systems too, i have combined
the mgeode case with the check for VMware.

Signed-off-by: Jeff Hansen <jhansen@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Alok N Kataria <akataria@xxxxxxxxxx>
Cc: Chris Snook <csnook@xxxxxxxxxx>

Documentation/kernel-parameters.txt | 7 +++++++
arch/x86/kernel/tsc.c | 33 +++++++++++++++++++++------------
2 files changed, 28 insertions(+), 12 deletions(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index e82c5bc..c5eb028 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2230,6 +2230,13 @@ and is between 256 and 4096 characters. It is defined in the file

+ tsc= Disable clocksource-must-verify flag for TSC.
+ Format: <string>
+ [x86] reliable: mark tsc clocksource as reliable, this
+ disables clocksource verification at runtime.
+ Used to enable high-resolution timer mode on older
+ hardware, and in virtualized environment.
+[2|3]= [HW,JOY]
TurboGraFX parallel port interface
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 4ae207c..24eff09 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -32,6 +32,7 @@ static int tsc_unstable;
erroneous rdtsc usage on !cpu_has_tsc processors */
static int tsc_disabled = -1;

+static int tsc_clocksource_reliable;
* Scheduler clock - returns current time in nanosec units.
@@ -99,6 +100,15 @@ int __init notsc_setup(char *str)

__setup("notsc", notsc_setup);

+static int __init tsc_setup(char *str)
+ if (!strcmp(str, "reliable"))
+ tsc_clocksource_reliable = 1;
+ return 1;
+__setup("tsc=", tsc_setup);
#define MAX_RETRIES 5
#define SMI_TRESHOLD 50000

@@ -745,24 +755,21 @@ static struct dmi_system_id __initdata bad_tsc_dmi_table[] = {

- * Geode_LX - the OLPC CPU has a possibly a very reliable TSC
- */
+static void __init check_system_tsc_reliable(void)
-/* RTSC counts during suspend */
+ /* RTSC counts during suspend */
#define RTSC_SUSP 0x100
-static void __init check_geode_tsc_reliable(void)
unsigned long res_low, res_high;

rdmsr_safe(MSR_GEODE_BUSCONT_CONF0, &res_low, &res_high);
+ /* Geode_LX - the OLPC CPU has a possibly a very reliable TSC */
if (res_low & RTSC_SUSP)
- clocksource_tsc.flags &= ~CLOCK_SOURCE_MUST_VERIFY;
-static inline void check_geode_tsc_reliable(void) { }
+ tsc_clocksource_reliable = 1;
+ if (boot_cpu_data.x86_hyper_vendor == X86_HYPER_VENDOR_VMWARE)
+ tsc_clocksource_reliable = 1;

* Make an educated guess if the TSC is trustworthy and synchronized
@@ -797,6 +804,8 @@ static void __init init_tsc_clocksource(void)
clocksource_tsc.mult = clocksource_khz2mult(tsc_khz,
+ if (tsc_clocksource_reliable)
+ clocksource_tsc.flags &= ~CLOCK_SOURCE_MUST_VERIFY;
/* lower the rating if we already know its unstable: */
if (check_tsc_unstable()) {
clocksource_tsc.rating = 0;
@@ -857,7 +866,7 @@ void __init tsc_init(void)
if (unsynchronized_tsc())
mark_tsc_unstable("TSCs unsynchronized");

- check_geode_tsc_reliable();
+ check_system_tsc_reliable();

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