[PATCH 2/2] virtio-blk: disable write cache if not negotiated

[Paolo Bonzini <pbonzini@redhat.com>]

If the guest does not support flushes, we should run in writethrough mode.
The setting is temporary until the next reset, so that for example the
BIOS will run in writethrough mode while Linux will run with a writeback
cache.

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 have
two bugs, 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.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 hw/virtio-blk.c |   14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/hw/virtio-blk.c b/hw/virtio-blk.c
index 280f96d..500e026 100644
--- a/hw/virtio-blk.c
+++ b/hw/virtio-blk.c
@@ -543,6 +543,19 @@ static uint32_t virtio_blk_get_features(VirtIODevice *vdev, uint32_t features)
     return features;
 }
 
+static void virtio_blk_set_status(VirtIODevice *vdev, uint8_t status)
+{
+    VirtIOBlock *s = to_virtio_blk(vdev);
+    uint32_t features;
+
+    if (!(status & VIRTIO_CONFIG_S_DRIVER_OK)) {
+        return;
+    }
+
+    features = vdev->guest_features;
+    bdrv_set_enable_write_cache(s->bs, !!(features & (1 << VIRTIO_BLK_F_WCE)));
+}
+
 static void virtio_blk_save(QEMUFile *f, void *opaque)
 {
     VirtIOBlock *s = opaque;
@@ -628,6 +641,7 @@ VirtIODevice *virtio_blk_init(DeviceState *dev, VirtIOBlkConf *blk)
     s->vdev.get_config = virtio_blk_update_config;
     s->vdev.set_config = virtio_blk_set_config;
     s->vdev.get_features = virtio_blk_get_features;
+    s->vdev.set_status = virtio_blk_set_status;
     s->vdev.reset = virtio_blk_reset;
     s->bs = blk->conf.bs;
     s->conf = &blk->conf;
-- 
1.7.10.2