[Qemu-devel] [PATCH] Only try and read a VMDescription if it should be there

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

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

The VMDescription section maybe after the EOF mark, the current code
does a 'qemu_get_byte' and either gets the header byte identifying the
description or an error (which it ignores).  Doing the 'get' upsets
RDMA which hangs on old machine types without the VMDescription.

Just avoid reading the VMDescription if we wouldn't send it.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
---
 migration/savevm.c | 37 ++++++++++++++++++++++++++++---------
 1 file changed, 28 insertions(+), 9 deletions(-)

diff --git a/migration/savevm.c b/migration/savevm.c
index 2091882..482529e 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -1125,16 +1125,35 @@ int qemu_loadvm_state(QEMUFile *f)
      * Try to read in the VMDESC section as well, so that dumping tools that
      * intercept our migration stream have the chance to see it.
      */
-    if (qemu_get_byte(f) == QEMU_VM_VMDESCRIPTION) {
-        uint32_t size = qemu_get_be32(f);
-        uint8_t *buf = g_malloc(0x1000);
-
-        while (size > 0) {
-            uint32_t read_chunk = MIN(size, 0x1000);
-            qemu_get_buffer(f, buf, read_chunk);
-            size -= read_chunk;
+
+    /* We've got to be careful; if we don't read the data and just shut the fd
+     * then the sender can error if we close while it's still sending.
+     * We also mustn't read data that isn't there; some transports (RDMA)
+     * will stall waiting for that data when the source has already closed.
+     */
+    if (should_send_vmdesc()) {
+        uint8_t *buf;
+        uint32_t size;
+        section_type = qemu_get_byte(f);
+
+        if (section_type != QEMU_VM_VMDESCRIPTION) {
+            error_report("Expected vmdescription section, but got %d",
+                         section_type);
+            /*
+             * It doesn't seem worth failing at this point since
+             * we apparently have an otherwise valid VM state
+             */
+        } else {
+            buf = g_malloc(0x1000);
+            size = qemu_get_be32(f);
+
+            while (size > 0) {
+                uint32_t read_chunk = MIN(size, 0x1000);
+                qemu_get_buffer(f, buf, read_chunk);
+                size -= read_chunk;
+            }
+            g_free(buf);
         }
-        g_free(buf);
     }
 
     cpu_synchronize_all_post_init();
-- 
2.4.3