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Re: [Qemu-devel] TSC frequency configuration & invtsc migration (was Re:

From: Paolo Bonzini
Subject: Re: [Qemu-devel] TSC frequency configuration & invtsc migration (was Re: [PATCH 4/4] kvm: Allow migration with invtsc)
Date: Tue, 10 Jan 2017 17:36:48 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.5.1

On 05/01/2017 11:48, Marcelo Tosatti wrote:
>> Host A has TSC scaling, host B doesn't have TSC scaling. We want
>> to be able to start the VM on host A, and migrate to B. In this
>> case, the only possible solution is to use B's frequency when
>> starting the VM. The QEMU process doesn't have enough information
>> to make that decision.
> That is a good point. But again, its a special case and 
> should be supported by -cpu xxx,tsc-frequency=zzzz.

I don't think this is a scenario that can work reliably.  The computed
TSC frequency may vary by 0.5% or so on every boot (e.g. you may get
2497000 kHz or 2511000 kHz for a 2.5 GHz TSC).  You can start the VM on
host A, reboot host B, and then you'll be unable to migrate.


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