libxc: provide notification of final checkpoint to restore end

When the restore code sees this notification it will restore the
currently in-progress checkpoint when it completes.

This allows the restore end to finish up without waiting for a
spurious timeout on the receive fd and thereby avoids unnecessary
error logging in the case of a successful migration or restore.

In the normal migration or restore case the first checkpoint is always
the last. For a rolling checkpoint (such as Remus) the notification is
currently unused but could be used in the future for example to
provide a controlled failover for reasons other than error

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Brendan Cully <brendan@cs.ubc.ca>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/libxc/xc_domain_restore.c

@@ -42,6 +42,7 @@ struct restore_ctx {
     xen_pfn_t *p2m; /* A table mapping each PFN to its new MFN. */
     xen_pfn_t *p2m_batch; /* A table of P2M mappings in the current region.  */
     int completed; /* Set when a consistent image is available */
+    int last_checkpoint; /* Set when we should commit to the current checkpoint when it completes. */
     struct domain_info_context dinfo;
 };
 
@@ -765,6 +766,11 @@ static int pagebuf_get_one(xc_interface *xch, struct restore_ctx *ctx,
         // DPRINTF("console pfn location: %llx\n", buf->console_pfn);
         return pagebuf_get_one(xch, ctx, buf, fd, dom);
 
+    case XC_SAVE_ID_LAST_CHECKPOINT:
+        ctx->last_checkpoint = 1;
+        // DPRINTF("last checkpoint indication received");
+        return pagebuf_get_one(xch, ctx, buf, fd, dom);
+
     default:
         if ( (count > MAX_BATCH_SIZE) || (count < 0) ) {
             ERROR("Max batch size exceeded (%d). Giving up.", count);
@@ -1296,10 +1302,23 @@ int xc_domain_restore(xc_interface *xch, int io_fd, uint32_t dom,
             goto out;
         }
         ctx->completed = 1;
-        /* shift into nonblocking mode for the remainder */
-        if ( (flags = fcntl(io_fd, F_GETFL,0)) < 0 )
-            flags = 0;
-        fcntl(io_fd, F_SETFL, flags | O_NONBLOCK);
+
+        /*
+         * If more checkpoints are expected then shift into
+         * nonblocking mode for the remainder.
+         */
+        if ( !ctx->last_checkpoint )
+        {
+            if ( (flags = fcntl(io_fd, F_GETFL,0)) < 0 )
+                flags = 0;
+            fcntl(io_fd, F_SETFL, flags | O_NONBLOCK);
+        }
+    }
+
+    if ( ctx->last_checkpoint )
+    {
+        // DPRINTF("Last checkpoint, finishing\n");
+        goto finish;
     }
 
     // DPRINTF("Buffered checkpoint\n");

tools/libxc/xc_domain_save.c

@@ -1616,6 +1616,20 @@ int xc_domain_save(xc_interface *xch, int io_fd, uint32_t dom, uint32_t max_iter
         }
     }
 
+    if ( !callbacks->checkpoint )
+    {
+        /*
+         * If this is not a checkpointed save then this must be the first and
+         * last checkpoint.
+         */
+        i = XC_SAVE_ID_LAST_CHECKPOINT;
+        if ( wrexact(io_fd, &i, sizeof(int)) )
+        {
+            PERROR("Error when writing last checkpoint chunk");
+            goto out;
+        }
+    }
+
     /* Zero terminate */
     i = 0;
     if ( wrexact(io_fd, &i, sizeof(int)) )

tools/libxc/xg_save_restore.h

@@ -131,6 +131,7 @@
 #define XC_SAVE_ID_TMEM_EXTRA         -6
 #define XC_SAVE_ID_TSC_INFO           -7
 #define XC_SAVE_ID_HVM_CONSOLE_PFN    -8 /* (HVM-only) */
+#define XC_SAVE_ID_LAST_CHECKPOINT    -9 /* Commit to restoring after completion of current iteration. */
 
 /*
 ** We process save/restore/migrate in batches of pages; the below