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

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 08/19/2013 01:22 AM, Paolo Bonzini wrote:
> Il 16/08/2013 11:49, Alexey Kardashevskiy ha scritto:
>> With KVM, we could fall back to the qemu implementation
>>> +     * when KVM doesn't support them, but that would be much slower
>>> +     * than just using the KVM implementations of the single TCE
>>> +     * hypercalls. */
>>> +    if (kvmppc_spapr_use_multitce()) {
>>> +        _FDT((fdt_property(fdt, "ibm,hypertas-functions", hypertas_propm,
>>> +                           sizeof(hypertas_propm))));
>>> +    } else {
>>> +        _FDT((fdt_property(fdt, "ibm,hypertas-functions", hypertas_prop,
>>> +                           sizeof(hypertas_prop))));
>>> +    }
> 
> This prevents migration from newer kernel to older kernel.  Can you
> ensure that the fallback to the QEMU implementation works, even though
> it is not used in practice?

How would it break? By having a device tree with "multi-tce" in it and not
having KVM PPC capability for that?

If this is the case, it will not prevent from migration as the "multi-tce"
feature is supported anyway by this patch. The only reason for not
advertising it to the guest is that the host kernel already has
acceleration for H_PUT_TCE (single page map/unmap) and advertising
"multi-tce" without having it in the host kernel (but only in QEMU) would
slow things down (but it still will work).


-- 
Alexey