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

[Paolo Bonzini]

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.

> 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.

Paolo