multifd: Be flexible about packet size

This way we can change the packet size in the future and everything
will work.  We choose an arbitrary big number (100 times configured
size) as a limit about how big we will reallocate.
Signed-off-by: NJuan Quintela <quintela@redhat.com>
Reviewed-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: NJuan Quintela <quintela@redhat.com>

migration/ram.c

@@ -786,13 +786,13 @@ static void multifd_pages_clear(MultiFDPages_t *pages)
 static void multifd_send_fill_packet(MultiFDSendParams *p)
 {
     MultiFDPacket_t *packet = p->packet;
-    uint32_t page_count = MULTIFD_PACKET_SIZE / qemu_target_page_size();
+    uint32_t page_max = MULTIFD_PACKET_SIZE / qemu_target_page_size();
     int i;
 
     packet->magic = cpu_to_be32(MULTIFD_MAGIC);
     packet->version = cpu_to_be32(MULTIFD_VERSION);
     packet->flags = cpu_to_be32(p->flags);
-    packet->pages_alloc = cpu_to_be32(page_count);
+    packet->pages_alloc = cpu_to_be32(page_max);
     packet->pages_used = cpu_to_be32(p->pages->used);
     packet->next_packet_size = cpu_to_be32(p->next_packet_size);
     packet->packet_num = cpu_to_be64(p->packet_num);
@@ -809,7 +809,7 @@ static void multifd_send_fill_packet(MultiFDSendParams *p)
 static int multifd_recv_unfill_packet(MultiFDRecvParams *p, Error **errp)
 {
     MultiFDPacket_t *packet = p->packet;
-    uint32_t page_count = MULTIFD_PACKET_SIZE / qemu_target_page_size();
+    uint32_t pages_max = MULTIFD_PACKET_SIZE / qemu_target_page_size();
     RAMBlock *block;
     int i;
 
@@ -832,12 +832,24 @@ static int multifd_recv_unfill_packet(MultiFDRecvParams *p, Error **errp)
     p->flags = be32_to_cpu(packet->flags);
 
     packet->pages_alloc = be32_to_cpu(packet->pages_alloc);
-    if (packet->pages_alloc > page_count) {
+    /*
+     * If we recevied a packet that is 100 times bigger than expected
+     * just stop migration.  It is a magic number.
+     */
+    if (packet->pages_alloc > pages_max * 100) {
         error_setg(errp, "multifd: received packet "
-                   "with size %d and expected maximum size %d",
-                   packet->pages_alloc, page_count) ;
+                   "with size %d and expected a maximum size of %d",
+                   packet->pages_alloc, pages_max * 100) ;
         return -1;
     }
+    /*
+     * We received a packet that is bigger than expected but inside
+     * reasonable limits (see previous comment).  Just reallocate.
+     */
+    if (packet->pages_alloc > p->pages->allocated) {
+        multifd_pages_clear(p->pages);
+        multifd_pages_init(packet->pages_alloc);
+    }
 
     p->pages->used = be32_to_cpu(packet->pages_used);
     if (p->pages->used > packet->pages_alloc) {