Am 19.08.2013 um 09:30 schrieb Alexey Kardashevskiy <a...@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).
It means that if you use the same QEMU version with the same command line on a
different kernel version, your guest looks different because we generate the
dtb differently.
The usual way to avoid this is to have a command line option to at least make
it possible for a management tool to nail down feature flags regardless of the
host configuration.
Considering that IIRC we haven't actually flagged -M pseries as backwards
compatible (avoid breaking migration, etc) we can probably get away with
enabling multi-tce always and live with the performance penalty on older host
kernels.
Alex
