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(-) -- 1.7.10.2 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
