On 12/19/2010 05:27 AM, Avi Kivity wrote:
On 12/17/2010 07:43 PM, Zachary Amsden wrote:
On 12/15/2010 10:16 AM, Julien Desfossez wrote:
Hi,
I'm currently working with the kvm clocksource and I'm wondering if
we could implement the vread function for this clock source when we
are running on a host with constant_tsc.
If I understand correctly the hv_clock structure is per_cpu because
of the eventual frequency changes, but in the case of constant_tsc
(and after validation that the TSC is synchronized across all the
cores) I think we could have a working vread function.
In case of migration, could we have a fallback in case we detect we
end up on a CPU without constant_tsc ?
Any advice/explanation would be greatly appreciated !
It's a bit more complex than that. In addition to the problem you
mention with migration, even if the TSC is synchronized, the kvmclock
still is not, even with constant_tsc. There is measurement error in
between reading the TSC and computing the per_cpu hv_clock offset
which varies between CPUs.
What about using rdtscp?
We could also disable kvmclock if constant_tsc and migration is not
desired, or if constant_tsc and the new tsc multiplier on bulldozers
is available on all machines in the migration cluster.
Even then, we need an atomic in the vread path.
The tsc multiplier does not look usable for virtualization, btw.
Zach
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