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 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
>
> 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 majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
