[Qemu-devel] [QEMU PATCH 0/2] virtio-blk: writeback cache enable improvements

[Paolo Bonzini <pbonzini@redhat.com>]

These patches let virtio-blk use the new support for toggling the cache
mode between writethrough and writeback.

The first patch introduces a new feature bit and configuration field to
do this.  The second patch disables writeback caching for guests that do
not negotiate VIRTIO_BLK_F_WCACHE (meaning that they cannot send flush
requests), so that limited or older guests are now safe wrt power losses.
VIRTIO_BLK_F_FLUSH has been introduced in Linux 2.6.32 (in 2009) and was
backported to RHEL/CentOS 5.6 (in 2010).

The Windows drivers (which work by emulating SCSI on top of virtio-blk)
have bugs in this area, which I reported on the Red Hat Bugzilla as
bugs 837321 and 837324.  With these patches they will suffer a
performance hit but gain correctness.

Paolo Bonzini (2):
  virtio-blk: support VIRTIO_BLK_F_CONFIG_WCE
  virtio-blk: disable write cache if not negotiated

 hw/virtio-blk.c |   30 ++++++++++++++++++++++++++++--
 hw/virtio-blk.h |    4 +++-
 2 files changed, 31 insertions(+), 3 deletions(-)

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1.7.10.2