[Qemu-devel] [PATCH v2 for-1.2] virtio-blk: hide VIRTIO_BLK_F_CONFIG_WCE from old machine types

[Stefan Hajnoczi]

QEMU has a policy of keeping a stable guest device ABI.  When new guest device
features are introduced they must not change hardware info seen by existing
guests.  This is important because operating systems or applications may
"fingerprint" the hardware and refuse to run when the hardware changes.  To
always get the latest guest device ABI, run with x86 machine type "pc".

This patch hides the new VIRTIO_BLK_F_CONFIG_WCE virtio feature bit from
existing machine types.  Only pc-1.2 and later will expose this feature
by default.

For more info on the VIRTIO_BLK_F_CONFIG_WCE feature bit, see:

  commit 13e3dce068773c971ff2f19d986378c55897c4a3
  Author: Paolo Bonzini <address@hidden>
  Date:   Thu Aug 9 16:07:19 2012 +0200

      virtio-blk: support VIRTIO_BLK_F_CONFIG_WCE

      Also rename VIRTIO_BLK_F_WCACHE to VIRTIO_BLK_F_WCE for consistency with
      the spec.

      Signed-off-by: Paolo Bonzini <address@hidden>
      Signed-off-by: Kevin Wolf <address@hidden>

Anthony Liguori <address@hidden> reported:

  This broke qemu-test because it changed the pc-1.0 machine type:

  Setting guest RANDOM seed to 47167
  *** Running tests ***
  Running test /tests/finger-print.sh...                OK
  --- fingerprints/pc-1.0.x86_64        2011-12-18 13:08:40.000000000 -0600
  +++ fingerprint.txt   2012-08-12 13:30:48.000000000 -0500
  @@ -55,7 +55,7 @@
   /sys/bus/pci/devices/0000:00:06.0/subsystem_device=0x0002
   /sys/bus/pci/devices/0000:00:06.0/class=0x010000
   /sys/bus/pci/devices/0000:00:06.0/revision=0x00
  -/sys/bus/pci/devices/0000:00:06.0/virtio/host-features=0x710006d4
  +/sys/bus/pci/devices/0000:00:06.0/virtio/host-features=0x71000ed4
   /sys/class/dmi/id/bios_vendor=Bochs
   /sys/class/dmi/id/bios_date=01/01/2007
   /sys/class/dmi/id/bios_version=Bochs
  Guest fingerprint changed for pc-1.0!

Reported-by: Anthony Liguori <address@hidden>
Signed-off-by: Stefan Hajnoczi <address@hidden>
---
v2:
 * Use DEFINE_VIRTIO_BLK_FEATURES() and host_features [Paolo]

 hw/pc_piix.c    |    4 ++++
 hw/virtio-blk.c |    1 -
 hw/virtio-blk.h |    3 ++-
 3 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/hw/pc_piix.c b/hw/pc_piix.c
index 0c0096f..d68dbb2 100644
--- a/hw/pc_piix.c
+++ b/hw/pc_piix.c
@@ -375,6 +375,10 @@ static QEMUMachine pc_machine_v1_2 = {
             .driver   = "qxl",\
             .property = "vgamem_mb",\
             .value    = stringify(8),\
+        },{\
+            .driver   = "virtio-blk-pci",\
+            .property = "config-wce",\
+            .value    = "off",\
         }
 
 static QEMUMachine pc_machine_v1_1 = {
diff --git a/hw/virtio-blk.c b/hw/virtio-blk.c
index fd8fa90..6f6d172 100644
--- a/hw/virtio-blk.c
+++ b/hw/virtio-blk.c
@@ -533,7 +533,6 @@ static uint32_t virtio_blk_get_features(VirtIODevice *vdev, 
uint32_t features)
     features |= (1 << VIRTIO_BLK_F_BLK_SIZE);
     features |= (1 << VIRTIO_BLK_F_SCSI);
 
-    features |= (1 << VIRTIO_BLK_F_CONFIG_WCE);
     if (bdrv_enable_write_cache(s->bs))
         features |= (1 << VIRTIO_BLK_F_WCE);
 
diff --git a/hw/virtio-blk.h b/hw/virtio-blk.h
index 35834cf..651a000 100644
--- a/hw/virtio-blk.h
+++ b/hw/virtio-blk.h
@@ -107,6 +107,7 @@ struct VirtIOBlkConf
 };
 
 #define DEFINE_VIRTIO_BLK_FEATURES(_state, _field) \
-        DEFINE_VIRTIO_COMMON_FEATURES(_state, _field)
+        DEFINE_VIRTIO_COMMON_FEATURES(_state, _field), \
+        DEFINE_PROP_BIT("config-wce", _state, _field, VIRTIO_BLK_F_CONFIG_WCE, 
true)
 
 #endif
-- 
1.7.10.4