migration: Fix non-multiple of page size migration

Unfortunately it's legal to create a VM with a RAM size that's
not a multiple of the underlying host page or huge page size.
Recently I'd changed things to always send host sized pages,
and that breaks if we have say a 1025MB guest on 2MB hugepages.

Unfortunately we can't just make that illegal since it would break
migration from/to existing oddly configured VMs.

Symptom: qemu-system-x86_64: Illegal RAM offset 40100000
     as it transmits the fraction of the hugepage after the end
     of the RAMBlock (may also cause a crash on the source
     - possibly due to clearing bits after the bitmap)

Reported-by:  Yumei Huang <[email protected]>
Red Hat bug: https://bugzilla.redhat.com/show_bug.cgi?id=1449037

Signed-off-by: Dr. David Alan Gilbert <[email protected]>
Reviewed-by: Juan Quintela <[email protected]>
Signed-off-by: Juan Quintela <[email protected]>

migration/ram.c

@@ -1312,6 +1312,8 @@ static int ram_save_target_page(RAMState *rs, PageSearchStatus *pss,
  * a host page in which case the remainder of the hostpage is sent.
  * Only dirty target pages are sent. Note that the host page size may
  * be a huge page for this block.
+ * The saving stops at the boundary of the used_length of the block
+ * if the RAMBlock isn't a multiple of the host page size.
  *
  * Returns the number of pages written or negative on error
  *
@@ -1335,7 +1337,8 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss,
 
         pages += tmppages;
         pss->page++;
-    } while (pss->page & (pagesize_bits - 1));
+    } while ((pss->page & (pagesize_bits - 1)) &&
+             offset_in_ramblock(pss->block, pss->page << TARGET_PAGE_BITS));
 
     /* The offset we leave with is the last one we looked at */
     pss->page--;