You had said up the thread, why not opt all non-normal VMs into the new
behavior. It will work great for TDX. But why do SEV and others want this
automatically?
Because I want flexibility in KVM, i.e. I want to take the opportunity to try and
break away from KVM's godawful ABI. It might be a pipe dream, as keying off the
VM type obviously has similar risks to giving userspace a memslot flag. The one
sliver of hope is that the VM types really are quite new (though less so for SEV
and SEV-ES), whereas a memslot flag would be easily applied to existing VMs.
Btw, does the "zap-leaf-only" approach always have better performance,
assuming we have to hold MMU write lock for that?
Consider a huge memslot being deleted/moved.
If we can always have a better performance for "zap-leaf-only", then
instead of letting userspace to opt-in this feature, we perhaps can do
the opposite:
We always do the "zap-leaf-only" in KVM, but add a quirk for the VMs
that userspace know can have such bug and apply this quirk.
But again, I think it's just too overkill for TDX. We can just set the
ZAP_LEAF_ONLY flag for the slot when it is created in KVM.
