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