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

Zachary Amsden
2011-01-07T20:44:20Z

On 01/07/2011 12:48 AM, Marcelo Tosatti wrote:
> 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.
>

Yes, I looked at that, but it looked to me like a subsection was
intended for an optional feature which MUST be present on the
destination if the source is using the feature. This way, newer hosts
without the feature enabled can migrate to older hosts which do not
support the feature.

The TSC rate migration is slightly different; we may wish to migrate
from a host with the TSC rate feature enabled to a host which does not
support the TSC rate feature. This is exactly the current behavior, the
TSC rate will change on that migration, and I wanted to preserve that
behavior. I don't advise that mode of usage, but there may be use cases
for it and it should be decided by policy, not dictated by our feature set.

That said, I'm happy to remove the MSR if we truly don't want to support
that mode of usage.

Zach