On 13/03/2018 19:49, Eduardo Habkost wrote:
>>> Exactly, in other words these two options are part of the guest
>>> ABI, and QEMU promises to never make the guest ABI depend on the
>>> host hardware unless you're using "-cpu host".
>> This is not entirely true; while MAXPHYADDR is constant downstream
>> unless using "-cpu host", in practice that behavior is wrong and a guest
>> could misbehave if passed a MAXPHYADDR that is different from the host's.
>> I think this is the same, and management software will have to live with it.
> I think they are very far from being equivalent.
Right, I only meant to say that guest ABI actually does depend on the
host hardware, even outside of "-cpu host".
> But if you tell the guest the wrong C-bit location, guests are
> likely to rely on it and break. Migration between hosts with
> different C-bit locations won't work, will it?
It won't---but as long as the destination hosts fails fast when the
C-bit location is wrong, it's okay. What matters is that we don't run
guest code with the wrong C bit, as you noted.