On 6/18/2008 3:57:16 PM, john stultz wrote: > > On Wed, 2008-06-18 at 19:41 -0300, Marcelo Tosatti wrote: > > On Wed, Jun 18, 2008 at 04:42:40PM -0500, Anthony Liguori wrote: > > > Marcelo Tosatti wrote: > > > > On Wed, Jun 18, 2008 at 04:02:39PM -0500, Anthony Liguori wrote: > > > > > > > > > > > Have we yet determined why the TSC is so unstable in the first > > > > > > > place? In theory, it should be relatively stable on > > > > > > > single-node Intel and Barcelona chips. > > > > > > > > > > > > > If the host enters C2/C3, or changes CPU frequency, it > > > > > > becomes unreliable as a clocksource and there's no guarantee > > > > > > the guest will detect that. > > > > > > > > > > > On Intel, the TSC should be fixed-frequency for basically all > > > > > shipping processors supporting VT. Starting with 10h > > > > > (Barcelona), I believe AMD also has a fixed frequency TSC. > > > > > > > > > > > > > But still stops ticking in C2/C3 state, I suppose? > > > > > > > > > > I don't know for sure but the TSC is not tied to the CPU clock so > > > I would be surprised if it did. I think that that would defeat > > > the utility of a fixed-frequency TSC. > > > > Well, Linux assumes that TSC stops ticking on C2/C3. > > > > Section 18.10 of Intel says: > > > > "The specific processor configuration determines the behavior. > > Constant TSC behavior ensures that the duration of each clock tick > > is uniform and supports the use of the TSC as a wall clock timer > > even if the processor core changes frequency. This is the > > architectural behavior moving forward." > > > > However it does not mention C2/C3. > > > > Could someone confirm either way? > > My understanding: On most systems, the TSC halts in C3. C2 may also > halt the TSC, but that seems to depend on the BIOS.
TSC stops counting in the H/W C3. The C-states reported by BIOS may not necessarily be mapped to H/W C-states; C2 used by BIOS may be C3 for H/W. > > thanks > -john > > -- > To unsubscribe from this list: send the line "unsubscribe kvm" in the > body of a message to [EMAIL PROTECTED] More majordomo info at > http://vger.kernel.org/majordomo-info.html . Jun Nakajima | Intel Open Source Technology Center -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
