[PATCH] Re: x86_32 tsc/pit and hrtimers

[Jeff Hansen]

[X86] Add tsc=stable option for marking TSC as stable

This enables legacy hardware or VMs without HPET, LAPIC, or ACPI
timers to enter high-resolution timer mode.

Signed-off-by: Jeff Hansen <jhansen@xxxxxxxxxxxxxxxxxx>
---
 Documentation/kernel-parameters.txt |    4 ++++
 arch/x86/kernel/tsc_32.c            |   12 +++++++++++-
 arch/x86/kernel/tsc_64.c            |    9 +++++++++
 3 files changed, 24 insertions(+), 1 deletions(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 9611505..8488074 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2084,6 +2084,10 @@ and is between 256 and 4096 characters. It is defined in the file
 			Format:
 			<io>,<irq>,<dma>,<dma2>,<sb_io>,<sb_irq>,<sb_dma>,<mpu_io>,<mpu_irq>

+	tsc=		[X86-32,X86-64]
+			tsc=stable: Mark TSC clocksource as stable, enabling
+			        high-resolution timer mode on older hardware.
+
 	turbografx.map[2|3]=	[HW,JOY]
 			TurboGraFX parallel port interface
 			Format:
diff --git a/arch/x86/kernel/tsc_32.c b/arch/x86/kernel/tsc_32.c
index 65b7063..f6e8f71 100644
--- a/arch/x86/kernel/tsc_32.c
+++ b/arch/x86/kernel/tsc_32.c
@@ -49,6 +49,17 @@ static int __init tsc_setup(char *str)

 __setup("notsc", tsc_setup);

+static struct clocksource clocksource_tsc;
+
+static int __init tscx_setup(char *str)
+{
+	if (!strcmp(str, "stable"))
+		clocksource_tsc.flags &= ~CLOCK_SOURCE_MUST_VERIFY;
+	return 1;
+}
+
+__setup("tsc=", tscx_setup);
+
 /*
  * code to mark and check if the TSC is unstable
  * due to cpufreq or due to unsynced TSCs
@@ -287,7 +298,6 @@ core_initcall(cpufreq_tsc);
 /* clock source code */

 static unsigned long current_tsc_khz;
-static struct clocksource clocksource_tsc;

 /*
  * We compare the TSC to the cycle_last value in the clocksource
diff --git a/arch/x86/kernel/tsc_64.c b/arch/x86/kernel/tsc_64.c
index 1784b80..4a3e555 100644
--- a/arch/x86/kernel/tsc_64.c
+++ b/arch/x86/kernel/tsc_64.c
@@ -291,6 +291,15 @@ __setup("notsc", notsc_setup);

 static struct clocksource clocksource_tsc;

+static int __init tscx_setup(char *str)
+{
+	if (!strcmp(str, "stable"))
+		clocksource_tsc.flags &= ~CLOCK_SOURCE_MUST_VERIFY;
+	return 1;
+}
+
+__setup("tsc=", tscx_setup);
+
 /*
  * We compare the TSC to the cycle_last value in the clocksource
  * structure to avoid a nasty time-warp. This can be observed in a
--
1.5.6.4

---------------------------------------------------
"If someone's gotta do it, it might as well be me."

On Thu, 9 Oct 2008, Chris Snook wrote:

> Alok kataria wrote:
> >  On Thu, Oct 9, 2008 at 12:53 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> >  wrote:
> > >  On Thu, 9 Oct 2008, Jeff Hansen wrote:
> > > 
> > > >  OK, so are we all agreed that something like clocksource_trust=tsc 
> > > >  would be
> > > >  the best?
> > >  No, it's per affected device: tsc=trust or tsc=stable or whatever
> > >  unintuitive name we want to come up. And it is a modification to TSC
> > >  not to the clocksource layer.
> >
> >  Yep, this is cool. I too have a patch in my local tree which does a
> >  similar thing i have a tsc_reliable flag which is set right now only
> >  when we are running under a VMware hypervisor.
> >  Along with marking the no_verify flag for TSC, this patch of mine also
> >  skips the TSC synchornization checks.
> >
> >          The TSC synchronization loop which is run whenever a new cpu is
> >  brought up is not actually needed on systems which are known to have a
> >  reliable TSC. TSC between 2 cpus can be off by a marginal value on such
> >  systems and thats okay for timekeeping, since we do check for tsc going
> >  back in read_tsc.
> >
> >  Can this reasoning be included and synchronization skipped for all
> >  these systems with reliable aka trustworthy TSC's  ?
>
> In general, no.  Not all hardware/hypervisors behave this way, even when the 
> TSC is otherwise stable once synchronized.
>
> -- Chris
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