KVM: arm64: Enable support for guest_memfd backed memory

Now that the infrastructure is in place, enable guest_memfd for arm64.

* Select CONFIG_KVM_GUEST_MEMFD in KVM/arm64 Kconfig.

* Enforce KVM_MEMSLOT_GMEM_ONLY for guest_memfd on arm64: Ensure that
  guest_memfd-backed memory slots on arm64 are only supported if they
  are intended for shared memory use cases (i.e.,
  kvm_memslot_is_gmem_only() is true). This design reflects the current
  arm64 KVM ecosystem where guest_memfd is primarily being introduced
  for VMs that support shared memory.

Reviewed-by: James Houghton <jthoughton@google.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20250729225455.670324-21-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Author: Fuad Tabba

arch/arm64/kvm/Kconfig

@@ -37,6 +37,7 @@ menuconfig KVM
 	select HAVE_KVM_VCPU_RUN_PID_CHANGE
 	select SCHED_INFO
 	select GUEST_PERF_EVENTS if PERF_EVENTS
+	select KVM_GUEST_MEMFD
 	help
 	  Support hosting virtualized guest machines.
 

arch/arm64/kvm/mmu.c

@@ -2302,6 +2302,13 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
 	if ((new->base_gfn + new->npages) > (kvm_phys_size(&kvm->arch.mmu) >> PAGE_SHIFT))
 		return -EFAULT;
 
+	/*
+	 * Only support guest_memfd backed memslots with mappable memory, since
+	 * there aren't any CoCo VMs that support only private memory on arm64.
+	 */
+	if (kvm_slot_has_gmem(new) && !kvm_memslot_is_gmem_only(new))
+		return -EINVAL;
+
 	hva = new->userspace_addr;
 	reg_end = hva + (new->npages << PAGE_SHIFT);