Re: [PATCH] Skip tsc synchronization checks if CONSTANT_TSC bit isset.

From: Alok Kataria
Date: Wed Oct 22 2008 - 18:04:20 EST

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,

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.


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at