Re: [RFC]Enlarge the dalta of TSC match window from one second to five second

[Marcelo Tosatti <mtosatti@redhat.com>]

On Mon, Aug 11, 2014 at 03:41:00PM +0800, xiexiangyou wrote:
> hi,
> 
> In kvm_write_tsc() func of kvm, The TSCs will be synchronized unless the time diff of creating vcpus small than one second.
> However, In my enviroment, stress is large, the vcpu creating time is delay, sometimes the diff time between vcpu creating
> is more than one second. In this case, TSCs in VM are not the same with each other when it boot.
> (1)To solve the issue, should we enlarge the dalta of TSC match window from one second to five second?
> 
> as follows:
> 
> 	 * it's better to try to match offsets from the beginning.
>           */
> -	if (nsdiff < NSEC_PER_SEC &&
> +	if (nsdiff < 5 *NSEC_PER_SEC &&
> 	    vcpu->arch.virtual_tsc_khz == kvm->arch.last_tsc_khz) {
>  		if (!check_tsc_unstable()) {
> 
> (2)Another way to solve the issue: setting all VPUs' tsc_offset equal to the first boot VCPU's. So in special case, hotpluging VCPU,
> we can ensure TSC clocksource is stable.
> 
> Thanks.
> xiexiangyou
> 
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Its OK to increase the matching window to 5 seconds.

Please send a proper patch.