Re: [Qemu-devel] [PATCH] powerpc iommu: enable multiple TCE requests

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 08/20/2013 06:39 PM, Paolo Bonzini wrote:
> Il 20/08/2013 03:36, Alexey Kardashevskiy ha scritto:
>> Hm. Here we might have a problem like this is we decide to migrate from
>> QEMU with this patch running on modern kernel to QEMU without this patch
>> running on old kernel - for this we might want to be able to disable
>> "multi-tce" via machine options on newer kernels. Do we care enough to add
>> such a parameter or we just disable migration and that's it?
> 
> Upstream doesn't support migration to older QEMU.


Hm. That makes things simpler. Then I do not understand why we need
migration protocol versions as QEMU version in stronger version for
migration. Ah, offtopic.

>> This SandyBridge,enforce - what if the destination host running on old
>> kernel was run without this option - will the migration fail?
> 
> The destination machine will not even start.
>
>>> But in this case, you do not need this because the hypercall works if
>>> emulated by QEMU.  I like Alex's solution of making it universally
>>> available in the dtb.
>>
>> The solution would be good if we did not already have H_PUT_TCE accelerated
>> for emulated devices in the host kernel but we do have it.
> 
> The question is also whether you consider pSeries support complete
> enough to be production ready---and until you have versioned machine
> types I would say you don't.
> 
> If you still consider it somewhat experimental, I would do as Alex said:
> make newer QEMU on older KVM as slower, and that's it.


Sorry if I miss anything, but is not it what the patch already does? :)
If so, I'll repost this patch + traces rework tomorrow or so.


-- 
Alexey