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Re: [Qemu-devel] [RFC PATCH 00/20] Kemari for KVM v0.1


From: Fernando Luis Vázquez Cao
Subject: Re: [Qemu-devel] [RFC PATCH 00/20] Kemari for KVM v0.1
Date: Fri, 23 Apr 2010 16:36:44 +0900
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.1.9) Gecko/20100411 Icedove/3.0.4

On 04/23/2010 02:17 PM, Yoshiaki Tamura wrote:
> Dor Laor wrote:
[...]
>> Second, even if it wasn't the case, the tsc delta and kvmclock are
>> synchronized as part of the VM state so there is no use of trapping it
>> in the middle.
> 
> I should study the clock in KVM, but won't tsc get updated by the HW
> after migration?
> I was wondering the following case for example:
> 
> 1. The application on the guest calls rdtsc on host A.
> 2. The application uses rdtsc value for something.
> 3. Failover to host B.
> 4. The application on the guest replays the rdtsc call on host B.
> 5. If the rdtsc value is different between A and B, the application may
> get into trouble because of it.

Regarding the TSC, we need to guarantee that the guest sees a monotonic
TSC after migration, which can be achieved by adjusting the TSC offset properly.
Besides, we also need a trapping TSC, so that we can tackle the case where the
primary node and the standby node have different TSC frequencies.




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