On 08/03/2018 23:44, Eduardo Habkost wrote:
>> I think doing so will be an issue for the migration. Consider your above
>> use case, a SEV guest is running on EPYC with cbitpos=47 and if we
>> migrate to some $NEXT AMD CPU which uses need to use cbitpos=48 and we
>> will fail to resume the guest on destination after migrating.
> 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.


> In theory we could make QEMU choose the right values
> automatically if we document very clearly that the default
> behavior is unsafe.  But I would rather not take that risk and
> force management software to be aware of the gotchas involved in
> using SEV + live-migration.

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