linux-kernel - [PATCH] Re: x86_32 tsc/pit and hrtimers

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Date:	Fri, 10 Oct 2008 08:24:53 -0600 (MDT)
From:	Jeff Hansen <x@...fhansen.com>
To:	Chris Snook <csnook@...hat.com>
cc:	akataria@...are.com, Alok kataria <alokkataria1@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>
Subject: [PATCH] Re: x86_32 tsc/pit and hrtimers

This one is against 2.6.27.


[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@...daccess-inc.com>
---
  Documentation/kernel-parameters.txt |    4 ++++
  arch/x86/kernel/tsc.c               |   11 +++++++++++
  2 files changed, 15 insertions(+), 0 deletions(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 1150444..0528bcb 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2174,6 +2174,10 @@ 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.c b/arch/x86/kernel/tsc.c
index 8f98e9d..70e485e 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -98,6 +98,17 @@ int __init notsc_setup(char *str)

  __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);
+
  #define MAX_RETRIES     5
  #define SMI_TRESHOLD    50000

-- 
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, 2008-10-09 at 15:50 -0700, Chris Snook wrote:
>> >  Alok Kataria wrote:
>> > >  On Thu, 2008-10-09 at 14:03 -0700, Chris Snook wrote:
>> > > 
>> > >  I agree that in general this should be no, but since this is a
>> > >  commandline variable it will be normally set for only those systems
>> > >  which have only TSC as a option or know that the TSC is reliable.
>> > >  wouldn't doing this be ok for such systems ?
>> >  Hardware doesn't deliberately do any TSC synchronization, though you 
>> >  might get
>> >  it by accident in some configurations.  A VMware guest gets it for free 
>> >  thanks
>> >  to the hypervisor doing it in software, but we need to run the check 
>> >  when we're
>> >  booting on bare metal.
>>
>>  The TSC sync algorithm right now expects that TSC are perfectly in sync
>>  between cpus.
>>  But, the hardware doesn't deliberately do any synchronization, so we can
>>  have situations where TSC was (accidently ? )off by a marginal value
>>  during boot and as a result we mark TSC as unstable and don't use it as
>>  a clocksource at all. For systems like the ones Jeff is using wouldn't
>>  that be a problem. IOW, even though the TSC was *marginally* off during
>>  bootup it should still be used as a clocksource, since you have no other
>>  option, no ? 
>
> You seem to be conflating position and rate.  When we mark TSC as stable, 
> we're saying it will always advance at a known rate on all CPUs, but this 
> says nothing about the relative positions on the different CPUs.  That skew 
> can be huge on some hardware, not just marginal, so we still need to 
> synchronize them at boot time, even though we don't need to (and can't, in 
> this case) verify stability with another continuous clock source.
>
> -- Chris
>
>
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