linux-kernel - Re: [PATCH] Skip tsc synchronization checks if CONSTANT

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Message-Id: <1224713046.13953.40.camel@alok-dev1>
Date:	Wed, 22 Oct 2008 15:04:06 -0700
From:	Alok Kataria <akataria@...are.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	LKML <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Daniel Hecht <dhecht@...are.com>
Subject: Re: [PATCH] Skip tsc synchronization checks if CONSTANT_TSC bit is
	set.

On Wed, 2008-10-22 at 13:17 -0700, Andi Kleen wrote:
> > the sync check is there to check the _offset_ between CPUs. CONSTANT_TSC
> > is not a guarantee that the TSC will be coherent across all CPUs.
> 
> I cannot find a quote for it in the docs now, but iirc the AMD
> definition of the bit guarantees offset, aka full synchronization
> over the system.

I found this mail,
http://lkml.org/lkml/2005/11/4/173

If you look for,

-------------------------------------------------------------
Future TSC Directions and Solutions
===================================
Future AMD processors will provide a TSC that is P-state and
C-State invariant and unaffected by STPCLK-throttling.  This
will make  the TSC immune  to drift.  Because using  the TSC
for  fast  timer APIs  is  a  desirable  feature that  helps
performance,  AMD  has  defined  a CPUID  feature  bit  that
software   can   test   to   determine   if   the   TSC   is
invariant. Issuing a CPUID instruction with an %eax register
value of  0x8000_0007, on a  processor whose base  family is
0xF, returns "Advanced  Power Management Information" in the
%eax, %ebx, %ecx,  and %edx registers.  Bit 8  of the return
%edx is  the "TscInvariant" feature  flag which is  set when
TSC is P-state, C-state, and STPCLK-throttling invariant; it
is clear otherwise.
--------------------------------------------------------------

The third line does mention about invariant TSC being immune to drift.

Thanks,
Alok

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