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

[Sean Christopherson]

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.