Re: [PATCH v7 11/14] KVM: Register/unregister the guest private memory regions

On Tue, Aug 02, 2022, Sean Christopherson wrote:

I think we should avoid UNMAPPABLE even on the KVM side of things for the core
memslots functionality and instead be very literal, e.g.

	KVM_HAS_FD_BASED_MEMSLOTS
	KVM_MEM_FD_VALID

We'll still need KVM_HAS_USER_UNMAPPABLE_MEMORY, but it won't be tied directly to
the memslot.  Decoupling the two thingis will require a bit of extra work, but the
code impact should be quite small, e.g. explicitly query and propagate
MEMFILE_F_USER_INACCESSIBLE to kvm_memory_slot to track if a memslot can be private.
And unless I'm missing something, it won't require an additional memslot flag.
The biggest oddity (if we don't also add KVM_MEM_PRIVATE) is that KVM would
effectively ignore the hva for fd-based memslots for VM types that don't support
private memory, i.e. userspace can't opt out of using the fd-based backing, but that
doesn't seem like a deal breaker.

Hrm, but basing private memory on top of a generic FD_VALID would effectively require
shared memory to use hva-based memslots for confidential VMs.  That'd yield a very
weird API, e.g. non-confidential VMs could be backed entirely by fd-based memslots,
but confidential VMs would be forced to use hva-based memslots.

Ignore this idea for now.  If there's an actual use case for generic fd-based memory
then we'll want a separate flag, fd, and offset, i.e. that support could be added
independent of KVM_MEM_PRIVATE.