Re: [PATCH RFC v7 07/64] KVM: SEV: Handle KVM_HC_MAP_GPA_RANGE hypercall

[Sean Christopherson]

On Mon, Jan 16, 2023, Nikunj A. Dadhania wrote:
> On 13/01/23 21:47, Sean Christopherson wrote:
> > It's perfectly legal for userspace to create the private memslot in response
> > to a guest request.
> 
> Sean, did not understand this part, how could a memslot be created on a guest request?

KVM_HC_MAP_GPA_RANGE gets routed to host userspace, at that point userspace can
take any action it wants to satisfy the guest request.  E.g. a userspace+guest
setup could define memory as shared by default, and only create KVM_MEM_PRIVATE
memslots for memory that the guest explicitly requests to be mapped private.

I don't anticipate any real world use cases actually doing something like that,
but I also don't see any value in going out of our way to disallow it.  Normally
I like to be conservative when it comes to KVM's uAPI, e.g. allow the minimum
needed to support known use cases, but restricting KVM_HC_MAP_GPA_RANGE doesn't
actually achieve anything and just makes things more complex for KVM.  E.g. the
behavior is non-deterministic from KVM's perspective if a userspace memslots update
is in-progress.