On 07/13/2010 11:42 AM, David S. Ahern wrote:
On 07/13/10 15:15, Zachary Amsden wrote:
What prevents a vcpu from seeing its TSC go backwards, in case the first
write in the 5 second window is smaller than the victim vcpu's last
visible TSC value ?
Nothing, unfortunately. However, the TSC would already have to be out
of sync in order for the problem to occur. It can never happen in
normal circumstances on a stable hardware TSC except in one case;
migration. During the CPU state transfer phase of migration, however,
What about across processor sockets? Aren't CPUs brought up at different
points such that their TSCs start at different times?
Yes, that's called an unsynchronized TSC. In that case, the
compensation does the best it can based on time since the first TSC
write, but it will never be exact.
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