On 10/24/2012 05:13 PM, Marcelo Tosatti wrote: > KVM added a global variable to guarantee monotonicity in the guest. > One of the reasons for that is that the time between > > 1. ktime_get_ts(×pec); > 2. rdtscll(tsc); > > Is variable. That is, given a host with stable TSC, suppose that > two VCPUs read the same time via ktime_get_ts() above. > > The time required to execute 2. is not the same on those two instances > executing in different VCPUS (cache misses, interrupts...). > > If the TSC value that is used by the host to interpolate when > calculating the monotonic time is the same value used to calculate > the tsc_timestamp value stored in the pvclock data structure, and > a single <system_timestamp, tsc_timestamp> tuple is visible to all > vcpus simultaneously, this problem disappears. See comment on top > of pvclock_update_vm_gtod_copy for details. > > Monotonicity is then guaranteed by synchronicity of the host TSCs > and guest TSCs. > > Set TSC stable pvclock flag in that case, allowing the guest to read > clock from userspace. > > Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx> If you are using a master copy, with a stable host-side tsc, you can get rid of the normal REQ_CLOCK updates during vcpu load. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
