[Qemu-devel] [PATCH] Fix xbzrle vs last_sent_block update

["Dr. David Alan Gilbert (git)" <dgilbert@redhat.com>]

From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>

My fix (84e7b80a) replaced the last_sent_block update that I'd
removed earlier; however it was too aggressive in the xbzrle case.

save_xbzrle_page might return '0' to mean that the page didn't
need sending since it was the same as the last sent version;
in this case we can't update 'last_sent_block' since we didn't
actually send it.

Symptom: 'Illegal RAM offset 1018000' as we try and send a page
        to the wrong RAMBlock;  potentially that could be a data
        corruption if you were really unlucky.

Fixes: 84e7b80a05c0c44b90533c6cd2f1db5c932ccf77

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
---
 migration/ram.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/migration/ram.c b/migration/ram.c
index 1eb155a..0490f00 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -716,6 +716,9 @@ static int save_zero_page(QEMUFile *f, RAMBlock *block, ram_addr_t offset,
  * ram_save_page: Send the given page to the stream
  *
  * Returns: Number of pages written.
+ *          < 0 - error
+ *          >=0 - Number of pages written - this might legally be 0
+ *                if xbzrle noticed the page was the same.
  *
  * @f: QEMUFile where to send the data
  * @block: block that contains the page we want to send
@@ -1249,7 +1252,13 @@ static int ram_save_target_page(MigrationState *ms, QEMUFile *f,
         if (unsentmap) {
             clear_bit(dirty_ram_abs >> TARGET_PAGE_BITS, unsentmap);
         }
-        last_sent_block = block;
+        /* Only update last_sent_block if a block was actually sent; xbzrle
+         * might have decided the page was identical so didn't bother writing
+         * to the stream.
+         */
+        if (res > 0) {
+            last_sent_block = block;
+        }
     }
 
     return res;
-- 
2.5.0