Merge remote-tracking branch 'remotes/stefanha/tags/block-pull-request' into staging

# gpg: Signature made Wed 15 Mar 2017 05:05:04 GMT
# gpg:                using RSA key 0x9CA4ABB381AB73C8
# gpg: Good signature from "Stefan Hajnoczi <stefanha@redhat.com>"
# gpg:                 aka "Stefan Hajnoczi <stefanha@gmail.com>"
# Primary key fingerprint: 8695 A8BF D3F9 7CDA AC35  775A 9CA4 ABB3 81AB 73C8

* remotes/stefanha/tags/block-pull-request:
  os: don't corrupt pre-existing memory-backend data with prealloc
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

util/oslib-posix.c

@@ -361,7 +361,19 @@ static void *do_touch_pages(void *arg)
         memset_thread_failed = true;
     } else {
         for (i = 0; i < numpages; i++) {
-            memset(addr, 0, 1);
+            /*
+             * Read & write back the same value, so we don't
+             * corrupt existing user/app data that might be
+             * stored.
+             *
+             * 'volatile' to stop compiler optimizing this away
+             * to a no-op
+             *
+             * TODO: get a better solution from kernel so we
+             * don't need to write at all so we don't cause
+             * wear on the storage backing the region...
+             */
+            *(volatile char *)addr = *addr;
             addr += hpagesize;
         }
     }