vhost+postcopy: Send address back to qemu

We need a better way, but at the moment we need the address of the
mappings sent back to qemu so it can interpret the messages on the
userfaultfd it reads.

This is done as a 3 stage set:
   QEMU -> client
      set_mem_table

   mmap stuff, get addresses

   client -> qemu
       here are the addresses

   qemu -> client
       OK - now you can use them

That ensures that qemu has registered the new addresses in it's
userfault code before the client starts accessing them.

Note: We don't ask for the default 'ack' reply since we've got our own.

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>

1 files changed, 23 insertions, 1 deletions

diff --git a/contrib/libvhost-user/libvhost-user.c b/contrib/libvhost-user/libvhost-user.c
index 7c8cd5878e..6314549b65 100644
--- a/contrib/libvhost-user/libvhost-user.c
+++ b/contrib/libvhost-user/libvhost-user.c
@@ -491,10 +491,32 @@ vu_set_mem_table_exec_postcopy(VuDev *dev, VhostUserMsg *vmsg)
                    dev_region->mmap_addr);
         }
 
+        /* Return the address to QEMU so that it can translate the ufd
+         * fault addresses back.
+         */
+        msg_region->userspace_addr = (uintptr_t)(mmap_addr +
+                                                 dev_region->mmap_offset);
         close(vmsg->fds[i]);
     }
 
-    /* TODO: Get address back to QEMU */
+    /* Send the message back to qemu with the addresses filled in */
+    vmsg->fd_num = 0;
+    if (!vu_message_write(dev, dev->sock, vmsg)) {
+        vu_panic(dev, "failed to respond to set-mem-table for postcopy");
+        return false;
+    }
+
+    /* Wait for QEMU to confirm that it's registered the handler for the
+     * faults.
+     */
+    if (!vu_message_read(dev, dev->sock, vmsg) ||
+        vmsg->size != sizeof(vmsg->payload.u64) ||
+        vmsg->payload.u64 != 0) {
+        vu_panic(dev, "failed to receive valid ack for postcopy set-mem-table");
+        return false;
+    }
+
+    /* OK, now we can go and register the memory and generate faults */
     for (i = 0; i < dev->nregions; i++) {
         VuDevRegion *dev_region = &dev->regions[i];
 #ifdef UFFDIO_REGISTER