Messages in this thread | | | Subject | Re: [PATCH 0/2] RFC: Precise TSC migration | From | Paolo Bonzini <> | Date | Mon, 30 Nov 2020 17:59:08 +0100 |
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On 30/11/20 17:54, Andy Lutomirski wrote: >> I*do think* however that we should redefine KVM_CLOCK_TSC_STABLE >> in the documentation to state that it only guarantees invariance if the guest >> doesn't mess with its own TSC. >> >> Also I think we should consider enabling the X86_FEATURE_TSC_RELIABLE >> in the guest kernel, when kvm is detected to avoid the guest even from trying >> to sync TSC on newly hotplugged vCPUs. >> >> (The guest doesn't end up touching TSC_ADJUST usually, but it still might >> in some cases due to scheduling of guest vCPUs) >> >> (X86_FEATURE_TSC_RELIABLE short circuits tsc synchronization on CPU hotplug, >> and TSC clocksource watchdog, and the later we might want to keep). > If you're going to change the guest behavior to be more trusting of > the host, I think > the host should probably signal this to the guest using a new bit. >
Yes, a new CPUID bit takes longer to propagate to existing setups, but it is more future proof.
Paolo
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