Re: [KVM TSC trapping / migration 1/2] Add TSC trapping for SVM andVMX

From: Zachary Amsden
Date: Thu Jan 06 2011 - 06:30:52 EST

On 01/06/2011 12:41 AM, Alexander Graf wrote:
Am 06.01.2011 um 11:10 schrieb Zachary Amsden<zamsden@xxxxxxxxxx>:

Reasons to trap the TSC are numerous, but we want to avoid it as much
as possible for performance reasons.

We provide two conservative modes via modules parameters and userspace
hinting. First, the module can be loaded with "tsc_auto=1" as a module
parameter, which turns on conservative TSC trapping only when it is
required (when unstable TSC or faster KHZ CPU is detected).

For userspace hinting, we enable trapping only if necessary. Userspace
can hint that a VM needs a fixed frequency TSC, and also that SMP
stability will be required. In that case, we conservatively turn on
trapping when it is needed. In addition, users may now specify the
desired TSC rate at which to run. If this rate differs significantly
from the host rate, trapping will be enabled.

There is also an override control to allow TSC trapping to be turned on
or off unconditionally for testing.

We indicate to pvclock users that the TSC is being trapped, to allow
avoiding overhead and directly using RDTSCP (only for SVM). This
optimization is not yet implemented.
When migrating, the implementation could switch from non-trapped to trapped, making it less attractive. The guest however does not get notified about this change. Same for the other way around.

That's a policy decision to be made by the userspace agent. It's better than the current situation, where there is no control at all of TSC rate. Here, we're flexible either way.

Also note, moving to a faster processor, trapping kicks in... but the processor is faster, so no actual loss is noticed, and the problem corrects when the VM is power cycled.

Would it make sense to add a kvmclock interrupt to notify the guest of such a change?

kvmclock is immune to frequency changes, so it needs no interrupt, it just has a version controlled shared area, which is reset.
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