migration/ram.c: Fix migration with compress enabled

Since ec6f3ab9, migration with compress enabled was broken, because
the compress threads use a dummy QEMUFile which just acts as a
buffer and that commit accidentally changed it to use the outgoing
migration channel instead.

Fix this by using the dummy file again in the compress threads.

Signed-off-by: Lukas Straub <lukasstraub2@web.de>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

migration/ram.c

@@ -688,12 +688,11 @@ exit:
  * @offset: offset inside the block for the page
  *          in the lower bits, it contains flags
  */
-static size_t save_page_header(PageSearchStatus *pss, RAMBlock *block,
-                               ram_addr_t offset)
+static size_t save_page_header(PageSearchStatus *pss, QEMUFile *f,
+                               RAMBlock *block, ram_addr_t offset)
 {
     size_t size, len;
     bool same_block = (block == pss->last_sent_block);
-    QEMUFile *f = pss->pss_channel;
 
     if (same_block) {
         offset |= RAM_SAVE_FLAG_CONTINUE;
@@ -867,7 +866,7 @@ static int save_xbzrle_page(RAMState *rs, PageSearchStatus *pss,
     }
 
     /* Send XBZRLE based compressed page */
-    bytes_xbzrle = save_page_header(pss, block,
+    bytes_xbzrle = save_page_header(pss, pss->pss_channel, block,
                                     offset | RAM_SAVE_FLAG_XBZRLE);
     qemu_put_byte(file, ENCODING_FLAG_XBZRLE);
     qemu_put_be16(file, encoded_len);
@@ -1302,15 +1301,14 @@ void ram_release_page(const char *rbname, uint64_t offset)
  * @block: block that contains the page we want to send
  * @offset: offset inside the block for the page
  */
-static int save_zero_page_to_file(PageSearchStatus *pss,
+static int save_zero_page_to_file(PageSearchStatus *pss, QEMUFile *file,
                                   RAMBlock *block, ram_addr_t offset)
 {
     uint8_t *p = block->host + offset;
-    QEMUFile *file = pss->pss_channel;
     int len = 0;
 
     if (buffer_is_zero(p, TARGET_PAGE_SIZE)) {
-        len += save_page_header(pss, block, offset | RAM_SAVE_FLAG_ZERO);
+        len += save_page_header(pss, file, block, offset | RAM_SAVE_FLAG_ZERO);
         qemu_put_byte(file, 0);
         len += 1;
         ram_release_page(block->idstr, offset);
@@ -1327,10 +1325,10 @@ static int save_zero_page_to_file(PageSearchStatus *pss,
  * @block: block that contains the page we want to send
  * @offset: offset inside the block for the page
  */
-static int save_zero_page(PageSearchStatus *pss, RAMBlock *block,
+static int save_zero_page(PageSearchStatus *pss, QEMUFile *f, RAMBlock *block,
                           ram_addr_t offset)
 {
-    int len = save_zero_page_to_file(pss, block, offset);
+    int len = save_zero_page_to_file(pss, f, block, offset);
 
     if (len) {
         stat64_add(&ram_atomic_counters.duplicate, 1);
@@ -1394,7 +1392,7 @@ static int save_normal_page(PageSearchStatus *pss, RAMBlock *block,
 {
     QEMUFile *file = pss->pss_channel;
 
-    ram_transferred_add(save_page_header(pss, block,
+    ram_transferred_add(save_page_header(pss, pss->pss_channel, block,
                                          offset | RAM_SAVE_FLAG_PAGE));
     if (async) {
         qemu_put_buffer_async(file, buf, TARGET_PAGE_SIZE,
@@ -1473,11 +1471,11 @@ static bool do_compress_ram_page(QEMUFile *f, z_stream *stream, RAMBlock *block,
     uint8_t *p = block->host + offset;
     int ret;
 
-    if (save_zero_page_to_file(pss, block, offset)) {
+    if (save_zero_page_to_file(pss, f, block, offset)) {
         return true;
     }
 
-    save_page_header(pss, block, offset | RAM_SAVE_FLAG_COMPRESS_PAGE);
+    save_page_header(pss, f, block, offset | RAM_SAVE_FLAG_COMPRESS_PAGE);
 
     /*
      * copy it to a internal buffer to avoid it being modified by VM
@@ -2355,7 +2353,7 @@ static int ram_save_target_page_legacy(RAMState *rs, PageSearchStatus *pss)
         return 1;
     }
 
-    res = save_zero_page(pss, block, offset);
+    res = save_zero_page(pss, pss->pss_channel, block, offset);
     if (res > 0) {
         /* Must let xbzrle know, otherwise a previous (now 0'd) cached
          * page would be stale