On Tue, Oct 30, 2012 at 12:34:25PM +0400, Glauber Costa wrote: > 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. Yes. The updates are harmless and not frequent, though, so i'd rather not touch this. -- 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
