Bug 1459945

See also qemu-kvm bug 1452067.

1) start /usr/libexec/qemu-kvm -drive if=none,id=hd,file=$HOME/f25-64.qcow2 -device virtio-blk,drive=hd -m 256 --enable-kvm -serial pty -monitor stdio -M pc-i440fx-rhel7.0.0

2) start cat for pty opened by first QEMU instance, e.g. "cat /dev/pts/5" if it prints
     char device redirected to /dev/pts/5 (label serial0)

3) start /usr/libexec/qemu-kvm -drive if=none,id=hd,file=$HOME/f25-64.qcow2 -device virtio-blk,drive=hd -m 256 --enable-kvm -serial pty -monitor stdio -M pc-i440fx-rhel7.0.0 -incoming tcp:localhost:12345

4) start cat for pty opened by second QEMU instance

6) type "yes > /dev/ttyS0", endless stream of "y" comes out of cat instance #1

7) type ^Z to stop cat instance #1

8) start migration:
      migrate_set_speed 1G
      migrate tcp:localhost:12345

Expected results:
 endless stream of "y" should come out of cat instance #2

Actual results:
 qemu-system-x86_64: inconsistent state in serial device (tsr not empty, tsr_retry=0
 qemu-system-x86_64: Failed to load serial:state
 qemu-system-x86_64: error while loading state for instance 0x1 of device 'serial'
 qemu-system-x86_64: load of migration failed: Operation not permitted


Originally introduced by:

    commit 17f9a1880e63e32d9fff0cf821ba70ceed861da2
    Author: Dr. David Alan Gilbert <dgilbert>
    Date:   Wed Jun 24 13:39:56 2015 +0200

    Serial: Migration compatibility pre 2.2/7.2
    
    Message-id: <1435153196-26350-3-git-send-email-dgilbert>
    Patchwork-id: 66380
    O-Subject: [RHEL-7.2 qemu-kvm-rhev PATCH v3 2/2] Serial: Migration compatibility pre 2.2/7.2
    Bugzilla: 1215087
    RH-Acked-by: Juan Quintela <quintela>
    RH-Acked-by: Laszlo Ersek <lersek>
    RH-Acked-by: Michael S. Tsirkin <mst>
    RH-Acked-by: Paolo Bonzini <pbonzini>
    
    From: "Dr. David Alan Gilbert" <dgilbert>
    
    Disable subsections added in qemu 2.3
    
    Newer qemu fixed migration corner cases for serial by adding subsections,
    however if these are generated it will break backwards migration.
    Disabling these subsections on older machine types should leave it no
    worse than existing qemu, from which we're not aware of having any reports
    of problems, and still allow these improvements on new machine types.
    Even when a user isn't actively using a serial port a guest will
    probably initialise it and may send stuff (e.g. a copy of the console 
    messages or the login: prompt).
    
    Signed-off-by: Dr. David Alan Gilbert <dgilbert>
    Signed-off-by: Miroslav Rezanina <mrezanin>

The commit message mentions that "a guest will probably initialise it and may send stuff", but a disconnected console eats data immediately.  Only a hungup reader can cause data to accumulate in the serial port's FIFO.