x86: kvm: rate-limit global clock updates

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When we update a vcpu's local clock it may pick up an NTP correction.
We can't wait an indeterminate amount of time for other vcpus to pick
up that correction, so commit 0061d53daf introduced a global clock
update. However, we can't request a global clock update on every vcpu
load either (which is what happens if the tsc is marked as unstable).
The solution is to rate-limit the global clock updates. Marcelo
calculated that we should delay the global clock updates no more
than 0.1s as follows:

Assume an NTP correction c is applied to one vcpu, but not the other,
then in n seconds the delta of the vcpu system_timestamps will be
c * n. If we assume a correction of 500ppm (worst-case), then the two
vcpus will diverge 50us in 0.1s, which is a considerable amount.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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Andrew Jones 2014-02-28 12:52:54 +01:00 • committed by Paolo Bonzini

parent ccf9844e5d

commit 7e44e4495a

2 changed files with 24 additions and 4 deletions

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arch/x86/include/asm/kvm_host.h

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27

arch/x86/kvm/x86.c

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