KVM: Add memory slot versioning and use it to provide fast guest write interface

Keep track of memslots changes by keeping generation number in memslots
structure. Provide kvm_write_guest_cached() function that skips
gfn_to_hva() translation if memslots was not changed since previous
invocation.

Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arch/x86/kvm/x86.c

@@ -3190,6 +3190,7 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
 		}
 		memcpy(slots, kvm->memslots, sizeof(struct kvm_memslots));
 		slots->memslots[log->slot].dirty_bitmap = dirty_bitmap;
+		slots->generation++;
 
 		old_slots = kvm->memslots;
 		rcu_assign_pointer(kvm->memslots, slots);