KVM: add doc note about PIO/MMIO completion API

[Marcelo Tosatti <mtosatti@redhat.com>]

How's that? Feel free to upgrade it.

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Document that partially emulated instructions leave the guest state
unconsistent, and that the kernel must complete operations before
checking for pending signals.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

diff --git a/Documentation/kvm/api.txt b/Documentation/kvm/api.txt
index c6416a3..ec0e72b 100644
--- a/Documentation/kvm/api.txt
+++ b/Documentation/kvm/api.txt
@@ -820,6 +820,11 @@ 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 must first finish
+uncomplete operations and then check for pending signals.
+
 		/* KVM_EXIT_HYPERCALL */
 		struct {
 			__u64 nr;