Re: [Qemu-devel] [RFC] More robust migration

[Anthony Liguori <anthony@codemonkey.ws>]

Jamie Lokier wrote:
> Paul Brook wrote:
>   
>> I never said you need the same host. All the save/restore code
>> should be host independent. It should be possible to save state on
>> (say) i386 and restore on ppc64. Anything that prevents this is IMO
>> a bug.
>>
>> For KVM you're likely to need a cpu with at least as many features
>> as the old one, but that's the price you pay for using host hardware
>> features.
>>     
>
> I'd prefer the "host hardware features" to be an acceleration
> mechanism, than something which makes a VM dependent on the specific
> host it's running on.
>
> Can't KVM invoke QEMU's emulation capabilities for those things it
> cannot provide itself because of missing host abilities?
>   

In theory, software can do anything :-)  In practice, it's pretty nasty 
to mix TCG execution with direct execution.  TCG doesn't guarantee 
atomicity of translated instructions so if you had TCG running one VCPU 
and bare metal running the other badness could occur.  Plus, TCG has 
hidden state that needs to be synchronized between the two.

Yeah, you could halt all VCPUs and just run one in TCG, or some other 
equally hackish thing.  But the point is, it's not easy and since it's 
not a tremendously popular thing to do, noone's working on it.  We have 
much more important low-hanging fruit (like making qcow2 not suck).

Regards,

Anthony Liguori

> -- Jamie
>
>
>