New: Kvmclock: Time warp when hotplug vcpu.

https://bugzilla.kernel.org/show_bug.cgi?id=195207

Bug ID: 195207
Summary: Kvmclock: Time warp when hotplug vcpu.
Product: Virtualization
Version: unspecified
Kernel Version: ALL
Hardware: All
OS: Linux
Tree: Mainline
Status: NEW
Severity: normal
Priority: P1
Component: kvm
Assignee: [email protected]
Reporter: [email protected]
Regression: No

1. Both KVM and VM will use kvmclock asap, usually the use_master_clock is true, so all vcpus use the same base tsc and ns to calculate current ns.

2. When hotplug a vcpu to a VM, the use_master_clock will change to false, also KVM updates base tsc and ns to all vcpus periodically.

3. KVM uses clocksource_tsc to get current ns, while VM uses kvm_clock to get current ns. For the same delta_tsc, they will get different result.

4. After run for more than 10 hours on a 2.4Ghz host, a redhat 6.6 VM will panic when been hotpluged a vcpu. The reason is the time warp in VM timekeeping_get_ns
cycle_delta = (cycle_now - clock->cycle_last) & clock->mask; // cycle_now is less than clock->cycle_last

5. Redhat 6.7 doesn't have this issue when CONFIG_CLOCKSOURCE_VALIDATE_LAST_CYCLE=y
#ifdef CONFIG_CLOCKSOURCE_VALIDATE_LAST_CYCLE
static inline cycle_t clocksource_delta(cycle_t now, cycle_t last, cycle_t mask)

{
        cycle_t ret = (now - last) & mask;

        return (s64) ret > 0 ? ret : 0;
}
#else
static inline cycle_t clocksource_delta(cycle_t now, cycle_t last, cycle_t
mask)
{
        return (now - last) & mask;
}

#endif

6. It's better to fix it in KVM, when change base tsc and ns in kvmclock, also calculate ns using kvm_clock arithmetic, be sure no time warp.

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