Re: [KVM TSC trapping / migration 2/2] Add TSC KHZ MSR

Marcelo Tosatti
2011-01-07T10:48:59Z

On Thu, Jan 06, 2011 at 12:10:45AM -1000, Zachary Amsden wrote:
> Use an MSR to allow "soft" migration to hosts which do not support
> TSC trapping. Rather than make this a required element of any
> migration protocol, we allow the TSC rate to be exported as a data
> field (useful in its own right), but we also allow a one time write
> of the MSR during VM creation. The result is that for the common
> use case, no protocol change is required to communicate TSC rate
> to the receiving host.

Migration to hosts which do not support the feature can be achieved by
saving/restoring the TSC rate + flags in a subsection. A subsection
seems more appropriate than an MSR for this.

> This allows administrative tools to configure migration policy
> as they see appropriate. Rather than dictate this policy with the
> KVM implementation, we properly allow migration to hosts which both
> do and do not support setting of the TSC rate on the receiving end.
> If it is wished to not support migration to a host which lacks
> support for the TSC rate feature, that can be coordinated externally.
>
> Signed-off-by: Zachary Amsden <zamsden@redhat.com>