KVM: add doc note about PIO/MMIO completion API

Document that partially emulated instructions leave the guest state
inconsistent, and that the kernel will complete operations before
checking for pending signals.
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

Documentation/kvm/api.txt

@@ -820,6 +820,13 @@ executed a memory-mapped I/O instruction which could not be satisfied
 by kvm.  The 'data' member contains the written data if 'is_write' is
 true, and should be filled by application code otherwise.
 
+NOTE: For KVM_EXIT_IO and KVM_EXIT_MMIO, the corresponding operations
+are complete (and guest state is consistent) only after userspace has
+re-entered the kernel with KVM_RUN.  The kernel side will first finish
+incomplete operations and then check for pending signals.  Userspace
+can re-enter the guest with an unmasked signal pending to complete
+pending operations.
+
 		/* KVM_EXIT_HYPERCALL */
 		struct {
 			__u64 nr;