Re: Question on skip_emulated_instructions()

[Yoshiaki Tamura <tamura.yoshiaki@lab.ntt.co.jp>]

Avi Kivity wrote:
> On 04/08/2010 10:30 AM, Yoshiaki Tamura wrote:
>>
>> To answer your question, it should be possible to implement.
>> The down side is that after going into KVM to make the guest state to
>> consistent, we need to go back to qemu to actually transfer the guest,
>> and this bounce would introduce another overhead if I'm understanding
>> correctly.
>
> Yes. It should be around a microsecond or so, given you will issue I/O
> after this I don't think this will affect performance.

That is a good news.

>> And yes, all I need is some consistent state to resume VM from, which
>> must be able to continue I/O operations, like writing to disks and
>> sending ack over the network. If I can guarantee this, sending the VM
>> state after completing output is acceptable.
>>
>
> I suggest you start with this. If it turns out performance is severely
> impacted, we can revisit instruction completion. If performance is
> satisfactory, then we'll be able to run Kemari with older kernels.

I was almost to say yes here, but let me ask one more question.
BTW, thank you two for taking time for this discussion which isn't a topic on 
KVM itself.

If I transferred a VM after I/O operations, let's say the VM sent an TCP ACK to 
the client, and if a hardware failure occurred to the primary during the VM 
transferring *but the client received the TCP ACK*, the secondary will resume 
from the previous state, and it may need to receive some data from the client. 
However, because the client has already receiver TCP　ACK, it won't resend the 
data to the secondary.  It looks this data is going to be dropped.  Am I missing 
some point here?