From b1163e497bdb3d6f04593940b6ec7842ec35c8ff Mon Sep 17 00:00:00 2001
From: Paolo Bonzini <pbonzini@redhat.com>
Date: Thu, 11 Jan 2018 13:56:44 +0100
Subject: [PATCH 01/21] serial: always transmit send/receive buffers on
 migration

RH-Author: Paolo Bonzini <pbonzini@redhat.com>
Message-id: <20180111135644.16253-1-pbonzini@redhat.com>
Patchwork-id: 78551
O-Subject: [RHEL7.5 qemu-kvm-rhev PATCH v2] serial: always transmit send/receive buffers on migration
Bugzilla: 1459945
RH-Acked-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
RH-Acked-by: Laszlo Ersek <lersek@redhat.com>
RH-Acked-by: Miroslav Rezanina <mrezanin@redhat.com>

When subsections were added in qemu 2.3, they were all disabled
for machine types that require pre-2.3 compatibility.  The commit
message says "disabling these subsections on older machine types should
leave it no worse than existing qemu", but actually migrating from
new QEMU to new QEMU (but with old machine type) will detect an
inconsistent migration state and fail the migration:

   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 0x0 of device 'serial'

In fact, this shows that migration from new source to old destination
might have also eaten the data in the FIFO or transmit/receive buffers.

It's actually pretty easy to trigger the failure by connecting a console
to a hung-up reader (not a *disconnected* reader!).  The fix is to
handle the subsections the same as we did in the qemu-kvm BZ1452067.
The data registers are migrated, which may indeed cause some more migrations
to fail to old qemu-kvm-rhev, but it will fix migration to new qemu-kvm-rhev.
Some subsections are still keyed on migrate_pre_2_2; from the commit message
of downstream commit 7d2e8f9662feb64c0b15b6fd53e06e3c56921f27:

    thr_ipending can be reconstructed fairly
    reliably by serial_post_load.  The others are features that are
    unlikely to be used in RHEL, respectively receive timeout (Linux
    does not even have the UART_IIR_CTI symbol in the driver) and
    physical serial ports connected to a modem

I consider this okay because nobody has yet complained about it for
qemu-kvm.  It's also safer because the failure avoids serial data loss
on migration.  This is consistent with the intended use of subsections.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
---
 hw/char/serial.c | 12 ------------
 1 file changed, 12 deletions(-)

diff --git a/hw/char/serial.c b/hw/char/serial.c
index fe4f41d..d82f7eb 100644
--- a/hw/char/serial.c
+++ b/hw/char/serial.c
@@ -720,10 +720,6 @@ static const VMStateDescription vmstate_serial_thr_ipending = {
 static bool serial_tsr_needed(void *opaque)
 {
     SerialState *s = (SerialState *)opaque;
-    if (migrate_pre_2_2) {
-        return false;
-    }
-
     return s->tsr_retry != 0;
 }
 
@@ -743,10 +739,6 @@ static const VMStateDescription vmstate_serial_tsr = {
 static bool serial_recv_fifo_needed(void *opaque)
 {
     SerialState *s = (SerialState *)opaque;
-    if (migrate_pre_2_2) {
-        return false;
-    }
-
     return !fifo8_is_empty(&s->recv_fifo);
 
 }
@@ -765,10 +757,6 @@ static const VMStateDescription vmstate_serial_recv_fifo = {
 static bool serial_xmit_fifo_needed(void *opaque)
 {
     SerialState *s = (SerialState *)opaque;
-    if (migrate_pre_2_2) {
-        return false;
-    }
-
     return !fifo8_is_empty(&s->xmit_fifo);
 }
 
-- 
1.8.3.1