postcopy shared docs

Add some notes to the migration documentation for shared memory
postcopy.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

docs/devel/migration.rst

@@ -577,3 +577,44 @@ Postcopy now works with hugetlbfs backed memory:
      hugepages works well, however 1GB hugepages are likely to be problematic
      since it takes ~1 second to transfer a 1GB hugepage across a 10Gbps link,
      and until the full page is transferred the destination thread is blocked.
+
+Postcopy with shared memory
+---------------------------
+
+Postcopy migration with shared memory needs explicit support from the other
+processes that share memory and from QEMU. There are restrictions on the type of
+memory that userfault can support shared.
+
+The Linux kernel userfault support works on `/dev/shm` memory and on `hugetlbfs`
+(although the kernel doesn't provide an equivalent to `madvise(MADV_DONTNEED)`
+for hugetlbfs which may be a problem in some configurations).
+
+The vhost-user code in QEMU supports clients that have Postcopy support,
+and the `vhost-user-bridge` (in `tests/`) and the DPDK package have changes
+to support postcopy.
+
+The client needs to open a userfaultfd and register the areas
+of memory that it maps with userfault.  The client must then pass the
+userfaultfd back to QEMU together with a mapping table that allows
+fault addresses in the clients address space to be converted back to
+RAMBlock/offsets.  The client's userfaultfd is added to the postcopy
+fault-thread and page requests are made on behalf of the client by QEMU.
+QEMU performs 'wake' operations on the client's userfaultfd to allow it
+to continue after a page has arrived.
+
+.. note::
+  There are two future improvements that would be nice:
+    a) Some way to make QEMU ignorant of the addresses in the clients
+       address space
+    b) Avoiding the need for QEMU to perform ufd-wake calls after the
+       pages have arrived
+
+Retro-fitting postcopy to existing clients is possible:
+  a) A mechanism is needed for the registration with userfault as above,
+     and the registration needs to be coordinated with the phases of
+     postcopy.  In vhost-user extra messages are added to the existing
+     control channel.
+  b) Any thread that can block due to guest memory accesses must be
+     identified and the implication understood; for example if the
+     guest memory access is made while holding a lock then all other
+     threads waiting for that lock will also be blocked.